The execution of the diff program from the TFS client was reading in the

output as a unicode object, which worked a lot of the time, but was causing

problems in a few cases. This manifested itself most often when doing a diff of

a UTF-8 file which contains a BOM. These files are technically well-formed, but

they're rare in practice because UTF-8 has no need for BOMs. Somewhere in

reading in the output, the BOM in the diff was being converted to a series of

three bytes that weren't a UTF-8 BOM, and so trying to apply the patch later

would fail.
This also would manifest itself when trying to redirect the output of

rbt diff to a file.

Successfully did a diff of a UTF-8 file containing a BOM and Review Board was

able to apply the patch.