strange tube failure
 

when we do tube changes at our shop, we inspect the old tube to try to determine the cause of the flat. twice or thrice, i've come across something strange: a slit on the inside (rim side, not tire side) of the tube, about 1 cm long, about 1 cm from the valve stem, centered between and parallel to the seams. the rims had nothing out of the ordinary about them (no burrs or scratches), the last one even had a properly installed velox rim strip.
any other wrenches ever seen this, have any hypotheses?

you offended the gods. I've long since stopped trying to understand why flats happen. It's like asking why do bad things happen to good people.

Is the tube significantly smaller than the tyre?

And who was the person to install said tubes before it went flat and subsequently was delivered to your shop?

Pinched in the bead when inflated.Takes awhile to kill it.

this last one wasn't: 700Cx23-28 in a 700Cx25 tire. that is a good point, though would you think the over inflated tube would fail at one of the seams, rather between them?

i don't know about the previous ones, but the last one was installed by me. the tube was straight in the tire, which i mounted by hand.

true dat, but my experience with those types of pinch flats leads me to believe that they usually leave a distinctive star shaped hole, and make a big bang at their eventual demise (not the same as the "other" type, aka snakebites which tend to leave two small, parallel, straight slits and take no time at all to deflate)
come to think of it, a pinch flat caused by having to mount the tire with levers might leave a mark like that. but that too would deflate pretty quickly.

So this tube with the mystery slit goes flat slowly without a bang? Weird...

Examine the box the tube came in for cuts from box-cutters... maybe it went through the box enough to nick the tube, but not cut all the way through.

I've seen those. Inflate a tube outside the tire and some of them will have two bulges on either side of the valve stem. There is a stress riser caused by the double thickness of the rubber around the stem and also this area was exposed to heat when the valve stem was installed driving off the plasticizers. Add a bit of valve stem squirm cassed by braking use a tube slightly too small and a bit of dirt and this will be the first place the tube will fail.