What's the longest chain of orphan blocks? (How many confirmations have been necessary?)
The longest orphaned fork was created due the "value overflow" bug occurrence, ranging from block 74638 to 74691 (so 53 blocks). Such a situation is unlikely to ever repeat though.

Other than that, as far as I remember 4 blocks were overwritten by some strange behaving miner (related question), but I might be mistaken.

In general, 6 confirmations is enough in most cases to prevent any double-spend attempts, and 120 confirmations is enough to give the developers time to release a fix for some major bug that might occur before the coin creation transactions are mixed in with everything else, creating a big mess.