Let me share this story so you can all point fingers at me and laught at my newbie expense! 
So, I've just spent about an hour trying to track down a strange sprite problem:
I had some sprites moving around the screen, but sometimes it seemed like all sprites flickered off for one single frame!
After disabling more and more of my code, I finally had just the sprites moving about, moving nicely around the whole screen, all the way down, disappearing in the lower border and reappearing at the top, just as I intended (I bet most of you can guess what the problem is now! 
)
I finally came to the conclusion that somehow it seemed like when one sprite hit a certain position, all sprites turned off, regardless of what my program did.
That seemed crazy, but I soldiered on, manually changing the y-positon and recompiling. But I didn't have the patience to try all Y-values, so I thought, hey, perhaps the 9918-docs has something to say about this:
"Sprite processing is terminated if the VDP finds a value of 208 (D0) in the vertical position field of any entry in the Sprite Attribute Table.
DOH!
Guess the lesson to be learned is check the documentation before trying to find bugs! 
