IIS7 custom 404 giving different results

On IIS7, it is configured to throw custom error page on a 404. The odd thing is that you don't always get the custom 404 page but sometimes the default.

Here is an example. You can get 2 different results from what should be a single 404:

http://www.oregoncf.org/give.2/

http://www.oregoncf.org/give.2

http://www.oregoncf.org/give.

1st and 3rd give you the custom error page the 2nd gives you the default 404 error page, not the custom.

Anyone have an ideas of why this would be?