Things you just do once

[Paul]

Notice (while cycling) that your bar end plug is working its way out and, with a firm blow from the palm of your hand, bang it back in.
 

[border-rider]

Try to remove the leaf stuck in the spokes of your front wheel whilst still moving...

[Deano]

I've actually done that a few times, and never come a cropper.

The trick is, to judge the thwack correctly - strong enough to be effective, but not so strong that you end up scraping your chin along the road...

GWS

[Zoidburg]

Cleaning the crap out of the female connector on a vistalite battery.

The metal tweezers on your swiss army knife will get very hot in a split second if you manage to bridge the contacts.

[Wowbagger]

Should this be "Things you just do once" or "Things you do just once" or "Just things..." etc.?

There was a chap on the Dun Run once, or so I heard, who tried to adjust his front mudguard by kicking it while moving... Whatever happened to him?

[Chris S]

Attempt to freewheel on a fixed.

[Wowbagger]

Forget to reconnect the "noodle" from your V-brakes after you have put the wheel back in.

[Pingu]

Attempt to freewheel on a fixed.

Just once until the next time.

[Zoidburg]

Letting your attention wander when a fixed is in the workstand and the cranks are turning.

[Pancho]

Wonder if your front tyre is fully inflated and lean forward to squeeze it.

WTF did I do that for? My only excuse is that I was very, very tired.

[Chris S]

Drive a hundred miles to an audax, having packed two left shoes.

[Crumbling Nick]

Spend half an hour or more trying to find a leaf or some other debris under the front mudguard before discovering that the noise only happens when the rider in front stops pedalling.

[Jaded]

Attempt to freewheel on a fixed.

Attempt to freewheel on a track bike.

...on a corner at the velodrome

[mark]

Attempt to operate the right side thumb shifter with your left hand, while holding something in your right hand.

[rogerzilla]

Build, tension and true a wheel, and only then notice that it is 1 in 10 with right-handed rim drilling  

Not done for many years, but do you always check?  I usually assume it's left-handed drilling (by which I mean the spoke to the left of the valve hole is above the centreline, viewed from the hub).

[nightrider]

Drive 80 miles to a Audax 1 day early

[GrahamG]

Not notice that the spokes are crossing at the valve hole on a DIY wheel build until the it's time to pump the tyre up. Waiting for a dark and horrible winters day to sort that one out.

[TheLurker]

Lean forward to switch on a fork mounted lamp whilst descending a steep hill at speed whilst holding a battery carrier/restraint for a lorry ...

[DuncanM]

Try to pull your trousers out of the chain with your left hand.  Whilst riding a "stunt" BMX.

[border-rider]

Allow your shoe lace to get caught in the chain of a fixed wheel bike.

[toekneep]

Attempt to freewheel on a fixed.

I think we need a thread along the lines of "Things you keep on doing despite swearing you will never do it again" for this one.

[onb]

Try and check the air in your front wheel which looks a tad low by leaning over the bars as you cycle down the road.

[jogler]

Drive 80 miles to a Audax 1 day early

Similarly.I once arrived at the start a week early,but it was only 20-ish miles away

[andygates]

Think "I can make it through this canyon between two moving buses".   

[Basil]

Attempt to break wind quietly at work after a night of beer and curry.