So I'm overhauling a commuting bike belonging to one of my friends. Mainly going to install a new headset, because the bike's steering has been bad for awhile, and getting worse.
Now it looks like the bike needs a whole new fork.
The threads are totally worn down at one point around the steerer tube, probably from a mishandled lock-washer sometime in the bike's past. There is actually a hole on the front of the steerer tube! And the part of the tube above the band of worn-down threads isn't perfectly parallel to the part below.
Even when my friend first got the bike (built up at a local recyclery), it wasn't possible to cleanly adjust the headset - it would be tight at some points and loose at others when turning the front wheel.
My guess is that riding the bike with a headset that isn't perfectly tight at all angles of the front wheel, caused stress at that already-weakened spot on the fork, along with continued friction on that spot.
I can't confidently reconstruct the life course of this bike to explain the damage - there are multiple plausible stories - but this is crazy enough to be worth posting. And I'm interested to hear alternate possible explanations.
edit: I noticed that the steerer tube itself is bent, about 1/4 of the way up from the crown race. again, this may have been there before (and been the reason that the headset couldn't be adjusted ideally, causing all the rest of the carnage) or it could itself be a result of riding with a loose-at-some-angles headset.
Also, just to show where the thin section of the steerer tube is... the last picture shows that it starts just beneath the threads of the headset cup itself - everything (including both washers and the locknut) is screwed on as normal, locknut close to bottoming out. Makes it almost appear that the steerer tube was drawn (like a wire when put under tension and it gets thinner while lengthening).
edit 2: as a couple of people have suggested, the most likely cause of the thinned/grooved area on the steerer tube threads, is that some loose bearings got stuck against the inside edge of the bearing cup (not the part against which bearings are supposed to sit) and over time, wore down the steerer tube. check out the last picture (#7) that I posted. It looks just like a badly pitted bearing track.
Note that this doesn't explain the crazily bent steerer tube. It looks a whole lot worse when holding and handling the fork, than it does in the pictures.