I'm on my second inner-tube in as many weeks and the culprit seems to be the same between them; these diagonal marks on the side around the whole of it—but only on one side (sorry, didn't make a note of what side that was when removing it
:(). Some are shallower, some deeper; the second picture below is the one that was actually leaking.
When I took it in to my LBS the guy said I pinched the tube with the tire bead when installing it, but I have a hard time imagining that I somehow managed to pinch the entire tube all along one side, in such a regular pattern (the marks repeat regularly, every 1"), twice in a row.
I wouldn't call myself a bike expert, but I have, from time-to-time, replaced tubes; did my front wheel and it's been fine for months. Did I just get lazy/cavalier in my tube installs or is there some other explanation?