Building a uni up.
wheel in, bolts tightened in a cross pattern to ensure they all go on evenly. Loctited up so they don’t come undone prematurely, wheel spins freely, tire does not rub on frame…
because the tire is still sitting on the floor.
D’oh!
Gazz off rim. No problem. Gazz back on rim. Painful but not impossible. I’ve heard it was so hard. What’s that arrow on the tire? Oh…direction of rotation…pointing backward. Well…it’s an UPHILL tire.
Taking off the nubs on my KH cranks. Grounding nubs off complete. Use new-style bolt, but it sticks out a tiny bit because the shoulder on the underside of the bolt head hits the axle. Put the bolts on the ol’ lathe and take down the shoulder. Perfect, they are going to fit nice and snug. Screw the first one in, looks nice. Tighten it up… SNAP. The head broke right off. ARRRRGHH!! D’OH! Those shoulders needed to be there!!
Later,
Jess
Installing the cycle computer on the 29er for the first time; winding the wire round the leg of the fork I accidentally took it around a spoke. I spent ages getting the route of the wire right around the crown, only to have to take it all off again…
Phil
Set alarm for 6am, get up, shower, pack my gear into the car, drive to the base of the mountain to start shuttles up to the top with the other riders, put on leg armour, helmet, gloves, reach for unicycle…
…forgot to pack it. 
Andrew
Preparing to drive out to do some Muni riding I fill up my camel back and throw all my gear in the back of my friends SUV. I put the camel back on the floorboards by my feet. Upon arrival I notice my feet have been on the bite valve the entire drive and my camel back is now empty and this floor is VERY wet. There is no where near to fill it back up.
Take cranks off, paint em, put back on, the bolt wont go on all the way. Its on upside down…Another use for a bearing extractor, cranks that cant thread a crank remover.
I once had to put my wheel back on 6 times when shimming it in order to make it not rub… It would have been ok had i put it on the first time.
The biggest scar on my leg (except my knee scars that are each made from 10+ unicycle related mishaps) is from my metal pedal with pins on it. (anyone else got permanent red knees?)
As I was leaving my house, I accidentally kicked the wheel with my foot, it hit the door and stopped, I started to fall and dropped the unicycle instinctively. This was stopped by hitting the side of the shelf, and my forward monentum continued into a shin with much gravity behind it dropping onto the pedal.
Inside my house.
About to go for a ride.
Deepest one time scar yet.
After a hard nights riding, coming home along the streets through town. Rolling hop over a square covered manhole, top of seatpost breaks off long walk home
Repeat, same manhole, different seatpost.
Repeat, same manhole, snapped kh seat in half.
That manhole is cursed.
d’oh
Bend torker lx axle on 3 foot drop.
Month later decide to sell it.(to my brother,cheap)
Day before i sell it,do a 10 inch drop.
Snaps in half.
Day before i sell it.
Just HAD to ride it one more time.
D’oh
After learning ww, learn the ww 1ft. Get some result. And then made accidently the ww 0ft.
Man that one goes faaaast!
(my 1st ever fall of a unicycle long ago).
Installing Magura brake on a Hunter36 with a Scott Wallis seat (first time in this config). Notice that the brake line is a little short to slip over the holder. I’ll just stretch it a little and it will be perfect. Oops. Broke the (plastic) line - they aren’t all that strong. D’Oh.
—Nathan
I have a permanently pinkish and wrinkly right elbow…
Second day with my 5’ giraffe, trying to mount it from a rolling start (I’m guessing this isn’t called a rolling mount). I roll along pushing the giraffe in front, step up with my first leg, swing myself up to the top, then as my second foot comes up to meet the pedal it overshoots the mark and is now in front of it. I realise I’m going to fall, so I step off the first pedal…since my weight is back from vertical and I’m on top of the seat, this rolls the uni forwards and swings the second pedal forwards, grabbing the foot that missed and pushing it forwards. I fall with my upper body travelling backwards and my lower body travelling forwards from 5’ up and hit the concrete with my tailbone and right elbow. It hurt, but I found it quite funny.
Andrew
Out of all the times I have taken out my wheel to fix the tube, I always forget NOT to pump the tire back up before putting it back into the frame (because I have a brake). And the stem is short so its so hard to get the pump over the valve, plus the tire is soooo hard to take off, so I get pissed at that point. Anyone else done this?
…Do’h
way madly stoked, bottom of the mountain, all my biker friends are watching me, quick release, i tighten it. it comes off in my hand… damn
I got a new wheelset and forgot to put on any form of rim tape. Especially a bad idea when the tire is at 85 psi.
Go to an auto parts store and buy a valve extender. They’ll come in packs of 4, but what can ya do? You’ll end up with three more than you’ll need.
I pumped up my 29x2.1 NanoRaptor before I rode up to my weekly bridge game. About an hour into the game, suddenly the tire blew off the rim, which in the enclosed, echoing gym sounded like an explosion. I still haven’t lived that down.
I have never personally experienced one of these D’oh! moments while working on my unicycles. I have, however, witnessed a few. 
The best one that I can recall was David Poznanter pumping up an air seat. We were all gathered at Bruce Bundy’s house in Santa Cruz during the 1999 CA MUni Weekend. David was busy doing an air seat conversion on a Miyata saddle. He got the seat all put back together and gets ready to inflate it. He grabs the floor pump, puts the chuck on the valve, and raises the pump handle all the way up. Plunge it down a full stroke and then lift the handle all the way up again for another full stroke. Plunge the handle quickly down again for another full stroke and repeat one more time. Riiiip! go the seams of the Miyata seat cover. Oops! Air seats don’t take that much air to inflate. D’oh! I’m just glad it wasn’t me.