If you are on multiple channels, and the same user joins several of them, your script will whois them for each channel they join. Some servers may consider many whois's on the same person in a short time to be a flood. To prevent flooding like that, you can use a script like this:

on *:CONNECT: unset %wdelay.*
on *:JOIN:#:{
  if ($($+(%,wdelay.,$cid,.,$nick),2) != 1) {
    set -u10 $+(%,wdelay.,$cid,.,$nick) 1
    whois $nick $nick

This code will delay whois'ing the same person for 10 seconds.