There are a couple of parts in a furnace that can either be going bad, or just doing what they were supposed to do due to something happening at that moment. A dirty flame sensor can be intermittent, so cleaning it is essential, or it will become more frequent. Perhaps the pressure switch is going bad, or maybe lousy timing sent a draft down or through the flue, which can trick the switch into thinking it's not getting the right pressure. If that happens more than a couple times in a week (assuming no heavy winds), you may want to replace it before it goes all together. These are common occurrences that are easy to check or fix yourself.