How could a user with 1 reputation point leave a comment?
Converted to comment
marked as duplicate by jonsca, ChrisF, Shawn Chin, Al E., Martijn Pieters Sep 28 '12 at 14:20
This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.
The user attempted to post that as an answer, but it was automatically converted to a comment by the system.
There is some debate over whether or not this is something that should happen, since as you noted the user cannot leave comments otherwise (he can only write comments with 50 reputation except when the system converts his answers automatically), but it's currently the expected behaviour.