 On a maxillary molar, there is more than a 50% chance that the tooth has four canals instead of just three, but the fourth canal, often called a "mesio-buccal 2", tends to be very difficult to see and often requires special instruments and magnification in order to see it (most commonly found in first maxillary molars; studies have shown an average of 76% up to 96% of such teeth with the presence of an MB2 canal).This infected canal may cause a continued infection or "flare-up" of the tooth.Any tooth may have more canals than expected, and these canals may be missed when the root canal procedure is performed.