While playing Sequence and a player places his card down that he intends to play and then changes his mind before placing his chip down is that allowed? or does he have to play the card he showed? Also, what if he places his chip down in the right spot but plays the wrong card. Does that count? Does he get to switch out the card or place the chip on the spot of the card he played by mistake?
The vast majority of games - including, as far as I know, Sequence - do not include this level of detail in the game rules themselves; if you are playing in a competitive tournament, you should refer to the tournament organisers. If you're a friendly game (particularly as I note you've tagged the question house-rules), you should do whatever causes people to have the most fun.