Repetiveness.

When people who repeat the same thing to me over and over again.  I'm not talking about over weeks, months, or years.  I mean when people repeat the same thing five times in a row, because they think I didn't hear it the first time.  If you think I didn't hear you, then maybe you simply ask "Did you hear me?" Instead of repeating the same thing constantly.  Hearing the same thing over and over again tends to get annoying extremely fast.