My subpanel has neutral and ground all mixed together?
Four Wire Feeder
If the panel is supplied by a four wire feeder (hot, hot, neutral, ground), then yes, the ground and neutral should be separate in the second panel.
If the panel is GFCI protected, and fed by a four wire feeder. Mixing the neutrals and grounds in the panel, will almost certainly cause the GFCI to trip. By mixing grounds and neutrals, you've essentially bonded the grounding and neutral bus bars together. This causes neutral current to be split between the grounding and neutral conductors of the feeder. Which means that the GFCI device will not see enough current on the neutral, and will open the circuit.
Three Wire Feeder
If this is an older installation, where the panel is fed by a three wire feeder (hot, hot, neutral), then the neutral bus bar should be bonded to the ground bus bar. In this case, mixing grounds and neutrals is poor aesthetics, but not a code violation.
If the panel is GFCI protected, and fed by a three wire feeder. Mixing grounds and neutrals should not trip the GFCI.