To succeed a roll in Call of Cthulhu you have to meet or roll under your skill threshold, but rolls that are between 96-100 are fumbles (critical failures).
What happens when you roll under your skill threshold (of 99), but within the fumble range as well?
It is a success (assuming a regular difficulty check).
From the top of Page 90 of the 7th edition core rulebook:
If the dice roll required for success is 50 or over and the dice read 100, a fumble has occurred.
If the dice roll required for success is below 50 and the dice read 96—100, a fumble has occurred.
