Possibly a really daft question this, so bear with me, but I went to a specialist running shop to get some trainers (New Balance 768, since you ask). I'm no great shakes on the running machine, so I tried them on and ran up and down the pavement a few times and all seemed fine. The shoes seemed comfortable in the short time I had to try them out, and I left the shop happy, if a little poorer.
So I went for my first run in the trainers, just a 5k to try them out, and I got a blister on the inside of my right heel. I suppose what I should have done is taken them straight back to the shop and complained, but I thought they'd settle down. The problem is, they haven't. I've probably done about 80k in the shoes now and instead of a blister, I'm getting a very nice callus, unless I run over 10k, in which case it's another blister.
The simple, daft question is: what would you do? I'd feel really stupid taking them back to the shop - they're dirty and clearly worn, but at the same time I feel really unhappy with the shoes - one foot is perfect, but the other is a nightmare.