Here's one possibility, if you've done DXM before.

http://www.erowid.org/experiences/exp.php?ID=24819

Also, Diazepam can cause a false possitive according to this. (supported by many other sites)

http://www.ultimatedetox.co.uk/drug-...-positives.htm

This one suggests taking Effexor causing a false PCP possitive.

http://www.bpinews.com/Breaking%20Ne...eakingnews.htm

I hope this helps you. good luck