Whether deliberately or otherwise...
Bath Green Park (BA61) - getting in a total mess with incoming trains that you end up sending a Jinty out on the Pines Express.
Tehachapi (TE91) or the BNSF Funnel - having a session where you don't care about scoring points and deliberately not moving anything for 6 - 8 hours just to sort out the resulting carnage. (Works a treat on other lines as well).
IS70 (Inverness) - sending a Class 47 to Wick!