It was a TYPO

I had a modulo sign where I should have had a division sign, but it's an instructive moment for two reasons (I enjoy bulleted lists, does that make me evil?):

• The reason I allowed such an elementary typo was that since I'm uncomfortable around unfamiliar or poorly-understood math (like our friend the modulus operator), my eyes will kind of glaze over and I'll just type in the code without paying too much attention. By contrast I never would have made that kind of mistake when, say, addressing a function with a bunch of parameters, I'd be on my toes syntax-wise. It's just another delightful little example of my math-related neuroses. 
• The only way I found the end of this tangle of Xmas lights was by applying the same logic I used on the collision problem: I need information about the source rectangle for the block on screen, and I need to be able to change that information. Since I can see several different blocks onscreen, it's unavoidable to realize I must already have the information I'm looking for, since I'm clearly using it to draw blocks. The bug hunt then becomes a relatively simple matter of finding the variable holding your info (in this case it belonged to a method within an instance of a burly Dictionary data structure, which I still don't quite get).