I've recently discovered that the following combinations of objects reveal bugs:
Bank:Object 17:1 with 4:15;
Bank:Object 17:1 with 4:12;
4:15 and 4:12 shoot fireballs at intervals. 17:1 contains instructions for these fireballs to change directions whenever it shoots its projectiles. This will cause 4:15's fireballs to suddenly change trajectory, always leftward. 4:12's fire balls will always move leftward, because it's firing interval matches that of 17:1.
These bugs are present in Knytt Stories 1.2.1, and most likely all earlier versions. Sorry if this has been reported it before, but Marzog's Run the Gauntlet: Part 1 is the first use I've seen of this buggy combination.