Ok I am fresh from disassembling and diagnosing this latest flat. Inspection showed a pinprick on the outside edge of the tube. Looking over the corresponding part of the tire, I found a small piece of sharp glass and removed it. Could this have caused the previous flat too but escaped my initial fix a few days prior? Possible, not sure.
But the larger question about flat frequency remains. I know my multiple posts already smack a bit of Mr. Orange, but I am going to list all my original theories, plus those suggested by commentators and evaluate each.
A. This is normal incidence - I should expect flat tires every so often with this much bike use. No, other commentators view this as wildly off base. GuitarStv goes 10,000 km in between flats.
B. Maybe I'm changing my tubes improperly - I'm shamefully ignorant on bike maintenance. Is there a wrong way to change a tire? I've watched a few videos and asked questions while the bike shop changed my tires in the past. My last change I didn't even use the two plastic tire levers to get the tire on and off. I just put the tube in place, wrestled the tire on, and then filled up at the gas station. That lasted only a month. Some recommend partially inflating the tube as you put it in place but I didn't do this. I've never patched a tube - I just replace. There were a few regrettable incidents in the past, but I'm cleaning up my act here and preinflating before I install the new tube.
C. Maybe I'm using cheapo Walmart tubes that don't hold up - In fact I am! Do you think these aren't good enough for normal commuting? I'm using $4 Duro tubes...but I have at least verified they are the correct diameter and width for my bike. Limited reviews online but maybe I'm going too cheap here.
People in this board don't seem to have problems with cheap tubes.
D. Maybe I'm riding over glass or sharp objects too much. One flat I attribute to a sharp stone that was unfortunately placed in my path, all the others I really don't know the cause. Ok, now we're getting warmer. But still, my commute is on asphalt and not discarded hypodermic syringes. There are occasional road hazards like glasss that I try to steer around, but I don't think I should be getting flats with this kind of frequency....right?
E. Maybe my tires themselves are bad/not resistant enough to puncture. I replaced them a year or so back with new but cheap tires. I think I paid $50 for the pair at a bike shop (but a place I trust) and installed myself. This is another possibility. I showed pictures of my worn front tire where the sidewall is damaged (probably from walking home with a flat, ironically). The rear tire doesn't show this kind of damage. Maybe the solution is gatorskins or whatever.
F. A spoke could be poking through and causing flats (suggested by GuitarStv, SimpleCycle and fatcow240). The rim tape seems to be in place. It feels smooth. The puncture holes I'm finding are on the outer perimeter of the tube, not the inner.
G. Sharp debris was still lodged in the tire (suggested by a good number of commentators). Yes, this is a strong possibility for the last two flats. However, it doesn't explain getting flats spaced out every month or so on both tires (unless there is some debris in both tires).
H. The tires are not inflated properly or the tubes are not sized correctly (suggested by RWD, sol, Dave1442397, bognish, frugaliknowit). Sizing wise I'm definitely ok. It's a 26" x 1.5" tire with a range of 60-90 psi. The tube packaging lists 26 x 1.25 - 1.75. The tube itself says 26 x 1.25.
Regarding pressure, there are times I could be running below 60 since I usually inflate to 70 and then let it go, sometimes up to a month. But if it's feeling sluggish I usually reinflate.
I. There could be burrs or other unique road hazards in my locale that cause trouble. (freeat57 suggestion)
I live in Connecticut and can't think of anything, aside from bumps in the asphalt and the occasional bit of sharp stuff on the road, that could be causing these flats. Certainly nothing unique to my location.