A prompt that does not get in the way

Yesterday, I saw a screenshot of a developer's box. His prompt was composed of two characters only: a colon and a semicolon (:;). This looks wired, doesn't it? Yes, it does, but there is a rationale behind it: using this prompt, you can copy and paste complete lines from a terminal into another one and they will just work. Why? Because those characters are ignored by the shell (the colon returns success and the semicolon starts another command).

Oh yes, this prompt does not provide useful information, but that's a matter of preference. E.g., I used a simple $ prompt for a long while, so :; is not any worse than it.