In the source code of the standard client,
checkpoints.cpp has this comment:
// What makes a good checkpoint block?
// + Is surrounded by blocks with reasonable timestamps
// (no blocks before with a timestamp after, none after with
// timestamp before)
// + Contains no strange transactions
Why should it matter? Presumably the client would have accepted it anyway if they were downloading it normally, and if you were to specify a later block, that would still force the client to download the strange block. I don't understand the point of this restriction.