I put a pork roast in the crock pot and accidentally cooked on warm for 30 minutes before I switched it to high! Is it safe to eat?
As seen in simplest form here, the FDA has deemed the temperature danger zone at between 41°F and 135°F inclusive.
Meanwhile, also in its simplest form here, we see again from the FDA that the internal temperature of pork must come to 145°F in order to be considered safe.
Finally, consider the following, as found here
Crock pots may vary but generally, the LOW setting is about 200 degrees F. and the HIGH setting is about 300 degrees F.
These three pieces of information in combined form make it certain that you're in no danger of having spoiled your cut of meat by cooking it only on Low for 30 minutes.
"High" doesn't turn the element on hotter, it just has a higher set point. Since many slow cookers can take over an hour to get the food inside up to temp, having it too low for the first 30 minutes has almost no difference on the temperature profile.
If you had it on "warm" for a couple of hours, that would be more problematic.
I'll leave the response, but the answer above isn't relevant for most units. It appears that only a very few high-end models have thermostats. For others, the settings are just different power settings. As such, warm would take much longer to bring a high mass of food up into the safe zone. (I don't think it would be a problem, but my answer no longer supports that)