TriMet monthly passes go on sale on the 20th of the month, so I always buy the upcoming pass while the current pass is still working. This means I put the new pass in my wallet behind the current month pass, and then forget about ever removing the out-of-date passes. 
Theoretically, this is $900 of TriMet transportation. But seeing as how each month is only valid for one month, and then becomes worth zero dollars, you are actually looking at a running total of $900, actual value: nada.
I think the first time I bought a monthly transit pass was for the T in Boston. It was probably 1997, and cost $35.00, or maybe $37.00?
Soon these types of passes will go the way of the dinosaur. TriMet is moving to a refillable card, something I’m not at all happy about.