Tips from Bourns on using trimming potentiometers. Link here (PDF)
Potentiometers normally have a small region at the extreme ends of the adjustment range where the output is irregular. This can be seen when there is a sudden drop to essentially zero resistance. Alternatively, the change in output will stop at some small residual value of resistance, depending on the design of the trimmer.
Although this unstable region is usually less than 1 % or 2 % of the total range of adjustment, it is important to remember to make allowances for avoiding it in your applications.