It is one of the hardest decisions an ale drinker will have to make. One of the problems with bitter is it has a penchant of going off much more regularly that any other form of alcohol. Which is why it is often preferable to drink in a pub which has a lot of ale drinkers, the turnover of barrels should be fast enough to prevent this problem arising. But even then after a slow day with a bit of air getting in, the beer can go off.

All good pubs will replace a pint of off beer. You are doing them a service after all, who wants to serve lousy beer (actually most pubs seem to want to serve lousy beer but that is a different issue). The problem is sometime identifying if a beer is off. You see the problem with real ale is that sometime a old sock smell, and a bitter sour aftertaste may well be what it is supposed taste like. This is officially when beer is on the turn.

This happened to me last night, and you have to talk yourself in to it actually being off. It becomes doubly difficult when it is the first time you have tried that beer. It took five sips from me and three from others for me to take it to the bar, not 100% sure that I was not making a fool of myself. But the barman replaced it without fuss and changed the barrel without even tasting it.

Well, if someone tells you that something tastes bad, do you really want to taste it yourself? (Human nature seems to suggest oddly, yes).