Brake wear.
Initially the RTO did not take into consideration multiple worn brakes.
Especially a situation where legally you can go with every brake worn to just above it limits.
There is brake wear limit indicated by an indicator on each brake.
The situation was highlighted by a DC10 that over ran on an RTO when it should have been able to stop.
( I am sure someone will fill in the details of the particular incident !
)
The problem was multiple brakes were worn but still in MM limits.
ADs ( 92-10-03, 97,02-04, etc) were issued that basically reduced the amount of wear that could be accepted and also consideration is given to uneven brake wear.