I never understand it when I see trolls come into forums, owned and managed daily by search engine representatives, and spam them. It is clear that these forums are crawling with people who can seriously impact your site's ranking in that search engine - but they are clueless to that fact?
Two recent examples for you:
(1) A Bing Forum thread discusses one member who has been just spamming the forum with link drops to his site. Literally every post of his, is a link to his site. The posts rarely answer questions related to the thread, they are just clogging up the forum.
One active webmaster finally called him out and then warned him:
And a side note, you really think it's a good idea to do this in the forum from a search engine company? I am sure they can add and remove URLs from the index if they really want this...
(2) Some guy came into the Google Webmaster Help forums and asked outright to do link exchanges. I mean, come on! Besides for Google clearly not approving such methods of link building, the forum is filled with people who absolutely hate exchanging links with others.
One of the main DO NOTs of SEO is DO NOT raise a red flag on your site. Coming into an official search engine forum that clearly is monitored by search reps and spamming it with links to your site, doesn't just raise a red flag, but gives them a bulls eye. Now, if you are doing this to a competitor, that is a different evil story.