Going off too soon

Street Chess saw a couple of games today, where one player was guilty of 'going off too soon' (also known as 'celebrating too early'). The diagram shows a final round game between Wenlin Yin and Yijun Zhang, after 8 moves of a Pirc. Zhang thought he saw a tactical trick and uncorked the TN 8. ... Nxe4? What made this move extra special was he pre-announced it with "Check this out" and followed it up with "Bang!" as he took the pawn. The early celebration turned to dismay when his opponent played 9.Qb5+, forcing off queens and breaking the pin of the knight. No matter how Black defended, losing a piece was going to be the end result.
(For reference the game started with 1.e4 d6 2.d4 Nf6 3.Nc3 g6 4.f4 Bg7 5.Nf3 c5 6.dxc5 Qa5 7.Qd4 dxc5 8.Qc4 Nxe4?? 9.Qb5+ Qxb5 10.Bxb5+ Bd7 11.Bxd7 Nxd7 12.Nxe4 and White won from here).
The other game was between Ian Hosking and Victor Braguine. Braguine (Black) had been attacking, but the position reduced to Q+B v Q+N and various pawns. Braguine grabbed a pawn, allowing his knight to be captured, and after giving a spite check, met by a king move, extended his hand. His opponent assumed it was a resignation (as I did I while watching the game). Suddenly Braguine picked up his Queen and gave another check, pushing the clock. When Ian asked him what he was doing he said "Sorry, I thought I had mated you". The game continued, and despite being down a piece, Braguine pulled off a later swindle to win the game.
(NB Hosking did not complain or ask for a ruling during or after the game. Clearly Braguine did not intend to resign, or even 'declare he resigned', so I would not have changed the result if asked).