If you never clean bottles with a brush, you can get gradual build up of beerstone on the bottom after a few cycles of use - visible when held up to light. It's then quite difficult to get out, even with a brush. I now give used bottles a good rinse under the tap after pouring a beer and then put them in the dishwasher. Then just sanitize the good ones on bottling day and separate out the ones with beerstone for brushing or another run through dishwasher.