- If its just a bit of more info (read:a paragraph), add it and leave a comment.
- If it's a completely new bundle of stuff, but is based on stuff gleaned from the answer, write an addendum answer as described above. Hat it with "This is in addition to @JonSkeet's answer". You may also comment on the original answer and ask the poster to incorporate your answer if s/he wishes--and then delete your answer once the deal is done (Something smilar could have happened here, though it didn't).
http://physics.stackexchange.com/questions/23824/will-an-emf-be-induced-across-the-ends-of-a-stationary-metal-rod-placed-in-a-tim/23895#23895Will an emf be induced across the ends of a stationary metal rod placed in a time-varying magnetic field?
In the case of wrong answers (or parts of answers), leave a comment. Try not to make radical changes in edits -- you can tell the OP in a comment to make them himself.