Its like frag reaper says, there aren't quotas, to put it simply when (some)people fail and think they shouldnt have they say its down to quotas. There may be more common times to pass/fail a test - I don't know. For example if you have an early morning/rush hour test chances are there will be more traffic on the road for you to deal with and more idiots rushing around. Therefore you might be more likely to fail than at 10/11:00 when the traffic has calmed down abit. Some test centres will have a higher failure rate than others, that doesn't mean they have a quota. It could be down to many things, the Nottingham one where I took my test is very busy, and has alot of hard routes where there is the potential for alot to go wrong. Compare that to a smaller centre with only a few routes, where you can pretty much 'learn' them all, and there will be less people taking their test and there will be differences in the pass rates.
Basically, there are NO QUOTAS. People just spread the rumours to make themselves feel better, its not their fault they failed, they were always going to fail because it was a friday afternoon and the examiner couldnt pass any more people that week.