The safemode status currently reports the number of unique reported blocks compared to the total number of blocks referenced by the namespace. However, it does not report the inverse: blocks which are reported by datanodes but not referenced by the namespace.
In the case that an admin accidentally starts up from an old image, this can be confusing: safemode and fsck will show "corrupt files", which are the files which actually have been deleted but got resurrected by restarting from the old image. This will convince them that they can safely force leave safemode and remove these files – after all, they know that those files should really have been deleted. However, they're not aware that leaving safemode will also unrecoverably delete a bunch of other block files which have been orphaned due to the namespace rollback.
I'd like to consider reporting something like: "900000 of expected 1000000 blocks have been reported. Additionally, 10000 blocks have been reported which do not correspond to any file in the namespace. Forcing exit of safemode will unrecoverably remove those data blocks"
Whether this statistic is also used for some kind of "inverse safe mode" is the logical next step, but just reporting it as a warning seems easy enough to accomplish and worth doing.