Many people are bad at remembering names, but are much worse with arbitrary numbers.
Several users here have names of the form
<$common string$><#arbitrary number#>.
Suppose I'm replying to a comment by user foo174328, and I begin typing
@foo, and being lazy, hit
<Tab> for autocomplete, and the autocomplete then gets confused because it doesn't know how to choose between users foo174327, foo17427, foo17432799, etc.
It would be nice to have SE recognize this annoyance, and now and then nag (or re-nag) applicable users to pick a handle that doesn't have so many numbers, and better still, algorithmically conforms to a certain "distance" between other names, the better to discourage such Too Many Daves problems.