One particular item of note was forgetting to clear the map data whenever a map was loaded into the editor, Polyspawn, which resulted in me loading in a half-finished copy of the map overtop of itself. The result was that a collision with a wall was processed twice, which made some walls (those which I had built earliest) to repel the test object with twice as much force. Naturally until I hunted it down there was the thought, "Why on earth do some walls make me fly back when I run into them!?".
Read the entry at the new Dev-Journal location.