I had a client last year have his envelope, check and voucher mangled at a Tennessee post office.  It was due 3/01.  Post Office sent it back to him in like little bitty pieces  (somehow they got his address).

It took us all year to convince the IRS that it was mailed in time even as we showed them the pieces and the postmaster explanation.  We had mailed a replacement check immediately and of course that one was late so they assessed late fees and penalties.