The current implementation of Stack Exchange allows us to comment on a particular question without even having enough reputation.
This can be done by exploiting the autocomment feature of Stack Exchange and then editing the comment as required.
Example: How do I convert a string to a string literal in python? (I posted and edited this comment when my reputation was below 50)
I think this is a very serious problem as one can abuse any question by creating new accounts with a script and commenting as much as he want.