I think the biggest surprise happened very early in the site's tenure, before most of the people we have now were active, hell, before I was active my first time: the Jack Crasher incident. There are still a few folks around from that time, so ask them if you want to know the whole story, but from what I understand Jack Crasher was an elite player at the end of his contract, and he elected to test free agency. When the free agency period opened all the teams reached out to him (this was well before the cap troubles we have today). Unbeknownst to all other teams, Crasher had agreed to sign with each one, but telling them not to mention anything or he would go back on his word.

And then, nothing. He left the site. I think this fits because it was a surprise to so many people. GMs of lesser teams were probably surprised he signed with them in the first place, all of the GMs were probably surprised when he dipped, as were many players no doubt. Definitely the biggest surprise I know of.