That, from time to time you get a missing piece and is very frustrating, expecially on the big puzzless.

What is this?

I've been playing this and aside from that it could do with more, and more varied music tracks the only real problem I've come across is that when I pick up a piece usually it should be click to pick it up, release it to drop it, but sometimes it doesn't drop it when I release the button, which is quite annoying....

From looking at the achievements, it looks like there are only 9 puzzles in different sizes?