From time to time there are reports in the local newspapers of newly arrived travelers being fleeced by illegal taxis at KLIA or KLIA2. The so-called taxi will charge something like RM300 or so for a one-way trip into KL. The stories cited cases "usually involving Middle Eastern tourists", but you never know.
If you have friends coming in from anywhere outside Malaysia into either airport, and nobody is picking them up, please let them know in advance not to take a taxi from anywhere else but the (official) taxi counter. Go find the counter, purchase the coupon there, and then go outside to the taxi waiting area (as guided by the staff of the taxi company).