I had my corned beef in the slow cooker on warm for 2 and a quarter hours before I saw it was not on high.  I then turned it to high and continued to cook it.  Is it still safe to eat? 
The rule of thumb is to never allow food to stay in the danger zone (40F-140F) for more than four hours - but that's total time, including service, unless you cook it long enough to pasteurize it. (Douglas Baldwin has pasteurization times for meats of various thicknesses at various temperatures, mostly for sous vide cooking, but you can still apply it to other cooking methods.) 