I'm usually bummed out by the lack of mathematical smoke and mirrors in d100 systems (don't tell me the odds!), but some games do interesting things with it. Like in Delta Green:
A critical success is a roll of 01 or any success where the dice match. So if your Agent’s skill is 50%, you achieve a critical success with a roll of 01, 11, 22, 33, or 44. A critical success automatically succeeds, and exceeds expectations.
A fumble is a roll of 00 (100) or any failure where the dice match. So if your Agent’s skill is 50%, you fum- ble on a roll of 55, 66, 77, 88, 99, or 00.