2TheApex: Expect multiple canals until proven otherwise.
Except for maxillary anteriors, all teeth should be expected to have more than one canal. Some commonly missed canals are MB2, the lingual canal in mandibular anterior teeth, second (or third!) canals in premolars, and a second distal canal in mandibular molars. Recently I had a very interesting case that was a new one for me: a second palatal canal in a maxillary molar.
Cone fit: Note palatal cone offset to the distal of the root.
Final: 4 canals, with 2 canals in the palatal root.
The palatal canals shared a common orifice, and the additional canal was not evident on initial access. The canals bifurcated apical to the orifice, and the second canal was located with the aid of the dental microscope to the mesial. A very interesting case, and a good reminder to always expect the unexpected.