So double 0 (or 00 + 0 = triple) on a pair of percentile dice indicates a roll of 100. In most games bigger rolls are usually better, but Call of Cthulhu uses a "roll under" (your skill or sanity or whatever target stat) system so a double 0 is the worst possible roll and is often regarded as a crit fail or fumble.

And since this is Call of Cthulhu... interesting times are ahead.