Pbarb2: Post a bug in the bug tracker with links to specific games and move numbers. Then Fencer may take a closer look.

Do you usually click on it before the page is completely loaded? And do you usually check the page before submiting the move to make sure that it was played where you intended it to? Sometimes, elements in the page may shift position while the page is being loaded, and you can click on one place that suddenly becomes some place else, and you end up making a move you didn't intend to. That's why you should always check the move before actually submiting.
Not sure if this is your case, but it's a frequent situation forsome users...