I checked my tire pressure prior to a ride yesterday and the tip of my Presta valve broke off. Long story short is that the air came out of my tire. Now I have all the supplies and know, in theory, how to replace a tube, however I have never had a flat before. So, I layer out everything, got my reading glasses on so I could see the details of what I was doing. There were a few surprises. It didn't occur to me to use the floor pump (duh) to put a little air into the new tube so it would more easily go into the tire, so I used a CO2 canister. I did not know that when the CO2 is expelled it sends out this white powdery stuff and it is very cold! No big deal, just didn't know it would happen. So then when I put the partially inflated tube into the tire the tube was waaaay too big! How could that be?! So I tried letting a little air out & guess what? It fit!! I put the tire & new tube easily onto the wheel, inflated, and reinstalled the wheel (front) onto my bike. Then I was worried that somehow it wasn't right. I went around the tire/new tube with my hands on both sides to ensure the bead of the tire was seated, on the inflated tire the small circumferential line ran parallel to the rim and the tube didn't bulge out, so everything should be good. Then I worried that somehow my wheel wasn't on fully, despite the fact that the brakes were equidistance from the rim with no rubbing, etc.
After everything was done I couldn't unscrew the lid to the CO2 device and had my neighbor unscrew it for me! I could have had it cross threaded but I don't think so.
NOTE: after initially cursing that my tubes were cheap pieces of cr*p as evidenced by the valve tips breaking off, I came to the realization that the tips (now noticed on the back also) were getting bent because of my pump hose "dismount" technique. I stopped by my LBS to buy new CO2 cartridge, tube, and have them eyeball my work and confirm that it was alright. I also got some tips regarding how to dismount the pump mount from your stem without bending the stem!