Sending addresses in the Address Book are not checked for validity.
Using the GUI, I added a space to some sending address (not on purpose) and for a long time, I could not figure out why its label was not displayed in my list of transactions.
I think those addresses should be checked for validity.

Using setaccount in bitcoind, validity of addresses are checked.
(There is another issue with setaccount, but I think it deserves a separate report)