I have accidentely put a whole leg of lamb in slow cooker and turned to warm. It took 6 hours until I noticed.
Is it safe to eat if I either cook on high or put in preheated oven?
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Stick a thermometer in it. If the temperature is over 140 F, the answer is "probably." However, there is no way to give you a definitive answer, because we cannot know the RATE at which the temperature rose. If it was below 140 F for more than about 3-4 hours cumulative in its entire history, it should absolutely be discarded. If there is anyone in your family at risk (pregnant, elderly, immune problems, and so on), it should definitely be discarded.
Sadly, there is a reason "When in doubt, throw it out" is second in food aphorisms only to "If you have time to lean, you have time to clean."
Take the safe road. Its only lamb and money--both can be replaced.