in reply to
Newbies, trying to help, and where to draw the line?
Having gone through the same experience of posting an answer to later find it was incorrect, I've developed what I think is a reasonable approach.
If you're not 100% sure then (when possible) test it yourself before you post.
1 of 2 things may happen:
1 - You come back to give the answer and find that someone else has already answered it.
No matter - you've learnt something new today and are better prepared to give advice in future.
2 - You come back to give your answer - the query's still unanswered - you can give a good answer and be assured it will be correct and useful.
/me let's put this in practise!