Much fun[1] can be had because RSA keys are integers. If your key would be encoded with a zero most significant byte, some implementations are fine if that byte is omited, and some will give a nonsensical error instead.
[1] Not actually fun, but I got a patch in OpenSSL, so I guess that's neat.
[1] Not actually fun, but I got a patch in OpenSSL, so I guess that's neat.