Here's one that took me a while to figure out: I was getting warnings for one particular user,
that they had solved
all available bonus puzzles (currently ~ 1200 puzzles in the pool),
even though their total was around 31000 points...
Turns out they
never solved a bonus puzzle, so every time they'd reach the bonus target,
the new target was shifted by only 25 points, and a new bonus puzzle selected..
Time to make it so you either keep your current bonus puzzle when this happens, or at the very least
the "lost" puzzle should be made available again.
edit: I've now changed it so unsolved bonus puzzles are returned to the pool of available puzzles.