Always have an open mind when confronted with tooth-ache.  Which tooth it is and how does the root canal system present.

Three roots in an upper first premolar is not common

Look for lesions that are biased to one side of a root. This is a hint that lateral or accessory canals are present and should be demonstrated following three dimensional obturation.

Second mesio-buccal canals in upper first molars are present 70% of the time, but differ in how they appear.