I recently saw an interesing bug: I have an account with reply address masking turned on. After a couple of back and forth exchanges with a correspondant, I got an email forwarded from SG with a damaged From field. This field, left uncorrected, would have prevented the mail from being sent.
My theory is that this occured because the name of my correspondant contains 8-bit chars (bloody Europeans, heh?) which got transcoded using the iso-8859-1 convention, and the resulting From line confused SG.
I put the damaged header and the sequence of events on the test server, in /home/sgtest/bad_headers/isochars.headers. Please take a look at it.