What error could occur if only a few oil drops were used, or if all the oil drops contained an even number of charges?
If all the drops contained an even number of electrons (e.g. 2,6,22, and 100), you might look at the charges you measured and determine that the fundamental unit of charge was $2e$ (rather than $e$ which would be the correct answer).
If you only did a few drops, you might have bad luck and get ones that are all divisible by numbers larger than $e$. (maybe you got charges of roughly $7e$, $49e$, and $98e$, all of which are divisible by $7e$).