In all of my years of mailing stuff back and forth with clients, I only had one envelope missing in action.  It was a client that had moved about 42 times in the last 2 years, including a move while the return was being prepared.  That was before the days of me using priority mail, so she got pissed that I couldn't tell her whether or not the envelope ended up in Haiti or wherever.  She was a real, true blue PITA client, so I send thank you cards every year to the USPS for helping me get rid of her.