Bbq pork ribs accidentally started cooking in crock pot on warm for 2 1/2 hours before noticed
It depends entirely on what the actual temperature for "warm" is.  If it is above 140F (i.e. outside of the "danger zone") you should be fine.

The limited research I've done on the "warm" temperatures of various crock pots seem to range from 145F to 200F, so it largely depends on the model.  You might want to read the manual or go to the manufacturer's web site and find out what the actual temperature for your specific model is.

However, if you can't actually find out what the warm temperature of your crock pot is, my usual rule with food safety is the proverbial "if in doubt, throw it out".  It's not worth the risk.