Heh, I've forgotten to take a pump a few times but never when I needed it. However, I've forgotten to take a 15mm wrench which I need to remove my folder's (commuter bike) rear wheel. DOH. I pumped the tire a couple of times and road the slow leak to the LBS. I was too upset at myself for forgetting the wrench to think of trying to patch the tube while still "on" the bike -- it can be done, you know -- i.e. off the wheel but still looped onto the bike. Oh well, no harm done that one time, and a lesson learned.

On a side note. I hit the mother of all potholes yesterday while riding up near Bear Mountain (heh, yeah, not commuting mind you) because I was frantically screaming to my brother and the other person ahead "CAR BACK!!!". Anyhow, first flat in ages and, what more, finally, after months of having it, my first chance to try my "new" mini-pump / CO2 combo device. You knew there had to be a connection to commuting in this story: a gear recommendation! While I don't know that I'd EVER be able to pump my tires to anywhere near what I'd like with that mini-pump, it would be an acceptable back-up if I were to run out of CO2, and it was perfect for plumping the tube before putting it on the rim. The CO2 inflation was perfect, using the trigger on the device. I can't recall the exact model name and Performance's site is wonky right now; otherwise I'd give a link... but I highly recommend it. It easily fits in a jersey pocket though I keep it on the water-bottle-holder attachment thingie on the road bike and toss it in the seatbag on my folder.

Edit: I forgot to add one more wrinkle, much in the spirit of this thread. The tube I pulled out of my road bike's seat bag to quickly replace the punctured tube -- not patch as I was riding with others and we wanted to get going -- was a 20" tube for my folder! I must have grabbed it by accident when last restocking my seatpack. Fortunately I had another in my bike backpack, as did my brother in his jersey pocket!