[ QUOTE ]
Dealer messes up.

[/ QUOTE ]

This is absolutely the dealer's fault.

"Player A" sat silently, and was as motionless as a cigar store Indian. The dealer, for reasons only he knows, decided to announce "check". Probably, a player at another table BEHIND Player A checked, prompting the dealer's erroneous announcement. Or perhaps the dealer was watching a hockey game on tv, and reacted audibly to a solid bodycheck. "CHECK!!!"

I can't think of any other reason why a dealer would announce "check". But one thing is for sure, there is no way that Player A absent-mindedly moved his hand while his brain was focused on trying to decide how much to bet on the end.

Later, when the out-of-turn bet was allowed to stand, that was also entirely the dealer's fault. "Player B" has no obligation to protect his hand, or his action. He is blameless.