I'm not sure if I'm using the right terminology but I know that theoretically it's possible for a block to be part of the longest (i.e. "main") blockchain but then be "replaced" by an alternative chain that is longer. Presumably it would most likely happen that the last block would be replaced in this way but it could also happen that the last 2, 3 or more could be replaced.
Are there any statistics on how often this actually happens? For example, the last block is replaced on average once a day, the last 2 blocks once every 10 days, etc?
This would have bearing on how confident one should be after a payment has had 2, 3, etc. confirmations.