That pops up occasionally for me. I had one where Bombus sandersoni, that is commonly described as having a thin yellow band on T5, but there is a large area in Canada where that species shows yellow on T4-6. So it had me wondering about the possibility of these being a color variation of Bombus frigidus. Two of us were looking through pictures to compare, and he pulled out specimens. In the end we concluded that this is the regional variation for sandersoni.
Another time, I was asked to review some observations for a project in Canada, and found an un-ID’d observation for Bombus ashtoni (very rare). On finding that, three of us were scouring unidentified observations across those provinces and looking at similar species. In the end, we found over a hundred ashtoni, and even a small pile of suckleyi (more rare).