Why do my tubes fail with a small area of rubber missing? My last three punctures weren't punctures at all (ie not a flint or thorn) but each time i looked at the tube it was like a small area of rubber had been rubbed away or just fell off. These are continental tubes. What could i be doing wrong when putting new tubes in? I always check the inside of the tyre too! The most recent case was a slow puncture, but the two before that were pretty quick.
It sounds like either something is rubbing a hole in the tube or it's a defective tube.

If it's a wear hole you might be able to locate what caused the wear hole.
Your wear point was either the tire, rim, or sometimes a tube gets folded and wears against itself.

I always put my tubes and tubes on so that the pressure rating on the tire is above the valve stem and on the freewheel side of the rear wheel or on the non-lever side for the front wheel. Doing this makes it easy to find pressure ratings and it allows me to more easily find where a hole in my tube came from.

Even if you don't know exactly how the tube was laying inside the tire looking at the hole in the tube in relation to the valve stem can help you find the wear spot.

Finding a rim wear point
If the hole is on the valve side of the tube in the narrow band that contacts the rim you can put the tube back on the rim and line up the hole with the rim to find the wear point. The wear point would be in one of two spots on either side of the valve hole in the rim. Look for something about the rim that might cause wear and correct it. Everything should be smooth and abrasion free.

Finding a tire wear point
If you know how the tube sat in the tire - because you put it on a specific way or because you remember then trace the hole back to the tire and look for a rough spot or something that could cause wear.
If you don't know how the tire sat in the tube feel around inside the tire and make sure everything is smooth. Be sure to check the inside of the tire bead.

Folded tube
If the hole is due to the tube rubbing on itself there are sometimes fold marks on the tube around the hole. When installing a tube make sure that it lies smoothly inside the tire with no creases or folds. 

Defective tube
If the tube is defective it will look like the tube material gets thinner around the hole or the seam is thinner than the rest of the material. If the hole looks rubbed or abraded then it's not a defect. Your local bike shop should have someone with an experienced eye who can identify defects.