mboxo? [1] It is a popular text format that cannot be unambiguously parsed.
More generally, most Unix tools' output is also not able to be unambiguously parsed. For example, use gcc to compile a file, then collect the warnings? The regex "^.+:\d+:\d+: warning.*" will be right most of the time, but there's no 'correct' way to parse gcc output (there is not a surjective mapping of output to input).
There are various ways to work around the problem: mboxrd format uses an escape sequence to work around the earlier problem mentioned with mboxo. `ls -l --dired' (GNU) will allow you to parse ls by appending filename byte offsets to the output. `wc --libxo xml` (FreeBSD) will give the output in XML, which is unambiguous as well. multipart/form-data (RFC2388) is used to embed binary data in a text format, by using a byte sequence which doesn't appear in the data.
Binary formats present their own set of issues, but "accidentally unparseable" is more common in text-based formats (or ad-hoc text output).