I'm interested to see what y'all have figured out about the game. Here are a few tricks, secrets, and other other silly things I have found.
This is pretty straightforward, but you can jump very high or very far by taking items out of your inventory in mid-air and dropping them. This gives you up to six jumps if you start with a regular jump, a full inventory, and something in your hands.
It may not be immediately obvious, but it's actually possible to fly indefinitely with a blue or red bird. (I don't think it's possible with a yellow bird.) If you rapidly put the bird into your inventory, take it back out, flap a few times, and repeat, you can attain any height or travel horizontally without falling.
There are two secret shops that I know of. The first is in the upper left corner of zone L. You need an O key to open the door (remember that you can find this in the box at the end of the tutorial zone), and you can buy an 8-second "teleportation bomb" for $18 or a box containing the three most expensive guns for $40 (a $139 value!).
The second is in the upper left corner of zone H. No key is required. There you can buy a 2-second teleportation bomb for $16 or a portable teleporter for $30 (normally $70).
It's possible to steal from the shops by using a teleportation gun, bomb, or box knocked on its side to send their wares to your save room. However..................the price tag will still be attached and you'll have to buy them anyway! :P I wonder if the shop keepers still get paid.
(http://i.imgur.com/WR5Cpwe.png)
In addition to the blue, green, white, and black gems, there is a special rainbow gem worth $99 that may be found in at least two places. The easier one to get to is on a floating island near the upper left corner of zone A. (Just grab a bird use the flying trick to get there.) The location of the other is described in the next section because it involves other spoilers.
If you go all the way back to the very first screen of the game, there's a ledge to the left with a gun on it. If you bring three or four items in your inventory, you can chain jump and enter a secret passageway to the left. Continue to the left and you fall into a room containing several teleportation bombs, some gems (including one worth $99), and a gold key. The key seems to be able to unlock any door. I'm not sure if this is intended as a shortcut, perhaps for speed running purposes, or if there is a door somewhere that can only be unlocked with this key. Let me know if you find one!
Nifflas said on Twitter that the master key exists for speedrunning purposes. It was also absolutely necessary for the recently held The Village is Lava competition, which required getting both endings without entering the village or unlocking more than two of the ZUG doors.
As for secrets not mentioned here yet...
Holding enter for a few seconds in the save room will cause one of several possible items to appear. You can get some pretty rare stuff this way.
If you activate developer mode by pressing F12 in the menu, you can go to any place you wish, including a few screens not normally accessible.
Two of them only serve to quickly trigger the ending cutscenes for testing purposes, but there's also a whole chain of puzzles that appears to have been made to test some of the gameplay mechanics. They are actually legitimately challenging and use some tricks that aren't really utilized much in the main game.
I get the impression the whole save room system worked differently in early builds of the game though. These screen all have teleporters that don't work correctly. Anything you put in them can't be accessed later on.
While developer mode is active, various additional objects will appear in the game. Their purpose appears to be to allow level designers to test their creations more efficiently by allowing them to cheat.
You can freeze the countdown of active time bombs by putting them in your inventory. Could be useful on some occasions, such as if you accidentally teleported active bombs.
If you press the key for a free inventory slot while standing on an object, it will be transferred there instantly without having to pick it up first.
You can put coins into your inventory by using the aforementioned trick. However, you will still collect them when taking them out of it. This could still have some uses though, seeing how inventory items aren't lost upon death.
You can actually carry coins if you pick them up while having 99 coins, seeing how this renders you unable to use coin-granting items. Don't see much of a use for it in the main game, but you could design custom levels with this in mind.
The violet gun is so powerful it can even destroy skull blocks, therefore removing the need to sacrifice animals. Probably not too useful outside of the prototype area though (which actually requires doing this) unless you're trying to minimize kills. Actually, that would make for an interesting challenge run, wouldn't it?
There is a detector detector [sic] hidden on a floating platform in the village. Doesn't really serve much of a purpose though unless you find it before you know the locations of the other detectors.
Teleport lasers are inactive as long as you have 9 items in temporary storage. Disabling a teleport laser this way is a perfectly viable solution for one of the puzzles that give you a red bird.
If you have a green gun, you can use it on the gems in the shop to get a nice amount of easy money. When I beat the game, I always kept a spare green gun in my save room so that I could easily buy almost anything I might want after dying.
If you bump into a shopkeeper, they'll start jumping around like a normal NPC. They might even buy their own products! This is what the "Coins" and "Spawn Coins" values are for in the editor.
The Boxes shopkeeper is broke, a cruel commentary on society as we know it, and there's no chance of anyone else walking in.
Same with the Library shopkeeper.
The Guns shopkeeper carries 17 coins with him, but there's a 5% chance an additional NPC (carrying 20 coins) will walk through the entrance.
The Bombs shopkeeper carries 99 coins with her. No other NPCs will enter.
The Birds shopkeeper has 2 coins with her. There's a 3% chance that another shopkeeper will be in the room, also with 2 coins.
The Gems shopkeeper carries 14 coins. Nobody else will enter the room.
The Cubes shopkeeper has 10 coins. Nobody else enters.
The box smuggler (Illegal Box Shop) in Zone H does not have any coins, and nobody else will enter.
Same with the gun smuggler (Illegal Gun Shop) in Zone L.
Pick up a red or blue bird, flap as high as you can, and then throw it down. Since birds fall slower than you do, eventually you'll land on the bird in mid-air. Pick up the bird again and flap as high as you can. Repeat.
This takes advantage of a mechanic I call "mercy-jumping". When you transition from being on an object to being in mid-air, there is a small window of time during which you can still jump, which makes platforming slightly easier.
Do everything described in the slow variation, but when you land on the bird, pick it back up and jump as quickly as possible. If done right, you will have jumped in mid-air. Flap as high as you can and repeat.
And here's every example of an object's spawn chance being less than 100% (though I'll be leaving out NPCs unless they're carrying something interesting). I found these out with Developer Mode. They all seem very impractical for speed-running unless someone figures out how the RNG system works:
The top green cube right here (https://image.prntscr.com/image/SoJDgxmjRzytXkf1G140OQ.png) only has a 40% chance of spawning.
In the "credits" building, there is a 1.2% chance for a purple gun to spawn, a 0.6% chance for a green gun to spawn, and a 0.3% chance for a gray gun (teleporter gun) to spawn.
On the floating island with the meta-detector (https://image.prntscr.com/image/r-QobWNgRFqGecAoFN_xEg.png), there is a 10% chance for a gray gun to spawn.
There is a 0.1% chance a short blue door will spawn right next to the entrance. Said door leads to the yellow bird platform in Zone L.
There is a 1% chance that a black beetle will spawn right here (https://image.prntscr.com/image/q6mO3ttPSh_F4UoVmKTXSQ.png). This beetle will transform into a brown beetle, which will transform into a light-blue beetle, which will transform into another black beetle, which will transform into an active 1-second bomb.
There is a 5% chance that a black beetle will spawn right above the entrance. This beetle will transform into an active 9-second bomb.[/url]