The rule has a parse error due to the trailing comma and thw whole rule should be ignored but IE’s error handling isn’t so clever and applies the first part of the rule when it should have ignored it all.
Run your code through the w3c validators (css and html) and you will pick these errors up easily.
There must be someone at MS who’s in charge of “being fussy about trailing commas,” because the same thing crops up in Javascript. FF and Chrome happily ignore unnecessary trailing commas in certain contexts, but IE chokes on them in weird and unpredictable ways!