Any post correcting an error in another post will contain at least one error itself.
Named for a Usenet poster. It is Muphry's law generalised beyond editing, and it survives because the corrector writes quickly while feeling secure.
In practice
- A reply correcting somebody's grammar contains a misplaced apostrophe, and the reply took eleven seconds to write.
- A correction issued in haste because the corrector felt secure is exactly the condition under which errors are made.