I was just reading a thread in the BTF Facebook Membership Hub about mistakes that peole have made doing triathlon so thought it would be good for people to 'fess up here
For me:
Leaving my cleat covers on my bike shoes in T1 and it took me a while to work out why my shoes wouldn’t go into the pedals so had to faff about losing time. The problem was I didn’t do it just once - I did it 3x the same season ffs!!!.
Wimbleball 70.3, not inspecting bike out. If I had I would have seen it was uphill out of transition and might not have chosen to put my shoes on the bike. As it was, I did, tried to flying mount on an uphill, slowed down very quickly, had to start pedalling before I was ready, with one of my shoes upside down which pinged off as it hit the ground 
Will have to have a think about my own, but what first sprung to mind was this bike I saw in racking at Elsinore 70.3. Who knows if the person realised when setting up the following morning…
Coming out of transition at Blenheim I was having issues clipping, looked down and sorted it, looked up and realised I was about the hit the barrier. Couldn’t avoid but managed to side swipe hard and carry on. Bars were slightly wonky for the rest of the race.
My biggest mistake was starting 
Weymouth 2018
I thought I’d be a clever clogs and leave a couple of gels in my trainers.
Obviously I completely forgot. Had a very uncomfortable first couple of kms until they popped. Leaving sticky gel everywhere
Totally ruined a pair of Adidas Ultraboost in the process - even the washing machine couldn’t get them clean
You’ve reminded me! I did similar at a Hever Evening Sprint race in my second year of tri. I wasnt flying mounting back then, but the entry and exit roped off chute on the bike was pretty narrow, and I looked down to start my bike computer, veered slightly, handlebar caught the rope, and flying off I went.
Felt rather stupid, but thankfully it was a grassy area, so no harm done!
I’ve made loads but my most memorable was doing my second or third tri ever about 25yrs ago in Sydney. When I was in transition setting up I saw a guy that had his pedals on his bike!
I thought that was a brilliant idea and put mine on my bike, never having seen or done it before and headed to the swim.
Exiting T1, jumped on bike at the exit, which was pretty much straight onto the road course. I was busy looking down, wondering why my shoes were upside down, fiddled a bit, kept looking down and promptly crashed into the kerb on the other side of the road.
Much merriment from the marshalls, much swearing from me, I ripped them off the pedals, put them on normally and wen’t on my angry way. 
as a TO, I’ve lost count of the number of times I’ve had to stop athletes leaving transition with helmets on the wrong way round, or heading out on the run with a helmet on.
My first mistake was wearing a red orca elite Tri suit at a race. My mother in law mentioned afterwards that it had raised a few comments with a group of females she had stood next to.
My second was at the Bala standard tri I had the feeling that I putting out max effort on the bike but struggling to make any gains. 20 miles in I realised my brake callipers had been ‘adjusted’ so is was rubbing on the wheel the whole time. The thing is I checked it before leaving transition before the swim and I know others had mentioned something similar that day.
The other mistake was wearing goggles for the entire race as I put them around my neck coming out of the swim and they stayed there all day.
Carefully preparing all my nutrition for Challenge Roth the day before, then leaving it in the fridge on race day. Started the bike with no gels and about 200ml of fluids. 
Lucky that the aid stations are frequent and well stocked, so other than some pre race stress it didn’t really affect my race
Guilty as charged re helmet on the run. First marshal on run course shouted to me so I threw it to him and went back to collect it at the end of the race.
First Tri I ever did (Hereford), outdoor pool with transition right next to it. Out of the pool and on the bike in a few seconds but body hadn’t got used to being upright, next thing I knew I was lying on the floor with people looking down at me.
Got back on and finished but back was a mess from skidding down the road and the blood combined with water & sweat it looked like I’d been disembowelled for the rest of the race.
Two google related ones;
Offroad tri with a pool swim and c200m run to transition. Decided I’d look cooler running down to T1 with my goggles round my neck rather than in my hand, and didn’t remember about them until about 5mins into the ride. Obviously I couldn’t afford the time to stop and take them off, so rode the whole thing with them bouncing around my throat.
Turned up at an olympic tri without either my goggles or my spare pair. Then the swim was moved from the open sea to a harbour, so it was a combination of salt water and boat fuel, couldn’t see properly for the rest of the day.
Hah, love it! Next step would be to wear a beaked plague mask and fill it will nutrition!
Here we go: nutrition mask 
aero wellies aero skin suit 
Other than ejecting water-bottles and spares, and other pretty standard errors, I remember running a bit late for a Thames Turbo race, havxing to park further away than expected, putting my bike together at the side of the road (1996 Fiesta, so both wheels off). Got to transition, set everything up, seemed fine.
Swim went OK.
Bike seemed fine, until I had to brake quite hard at a roundabout, I hadn’t tightened the rear QR skewer so the wheel popped out of the horizontal drop out, and wedged into the cut-away in the frame. I remember the marshal and I exchanging something along the lines of “What the fuck just happened?” as it took me a while to realise.
I stopped a guy leaving T1 at a Thorpe Park tri and asked him to tighten his QR. As he was running out I could hear his front wheel bumping up and down in the drop outs so guessed he hadn’t done the QR up, and I was right.
Turned up at the Cowman in Milton Keynes one year.
I hadn’t entered…
Racking the bike at Vitruvian in the morning, having travelled up the night before and staying in a hotel, decided to put it in an easier gear as I’d heard there was a hill at the start… Di2 battery dead.
I’d recently fitted the bluetooth dongle that gives you Di2 stats, including ironically the battery charge %. It seems that a firmware upgrade was required, otherwise it resulted in a very quick battery drain.
