Highest incidence of extra canal is seen in which tooth?
More than 70 percent of maxillary first molar has shown the occurrence of second mesiobuccal canal, and this is found to be the most common reason for retreatment of maxillary molars. 
If seen through Dental operating microscope(DOM), approx. 90-95% cases show extra mesiobuccal canal (MB2).
Maxillary 1st PM
Maxillary 1st molar
Maxillary 2nd molar
Mandibular 1st PM