Over on the ABA blog there's an article about how the longer you do birding, the harder it gets. I get what this article is talking about. Nothing drives me nuttier than a bird that deviates just a little bit from the norm.
The most memorable experience I've had misidentifying a bird (so far) happened while in High School. In December, shortly before Christmas break of my Junior year, I found a dead Swift on campus. I don't know how many thousands of Chimney swifts I'd seen at this time, but I knew I had something different in my hand. The Peterson field guide showed that a field mark for the Vaux Swift as a white throat, and in my hand I held a swift with a VERY WHITE THROAT. The guide even said they'd been spotted in Louisiana before, so I even thought that I had a good case for a vagrant swift. When I considered what I thought I had, I also had to consider, the bird was dead. It would never fly again. It didn't die because it was a pest, or because somebody wanted or needed to eat it. It struck a window. There's a lesson in there for another time.
I turned to the local birding expert, one of my teachers, to help me confirm the id. Dr. Hall has been involved in the Louisiana breeding bird census in the past, and has lead birding field trips around the area during what my high school called "special projects week". Our excitement bubbled, but he did give one word of caution. This would not just be highly unusual, this would be "third bird of its kind" in the state unusual. Our identification would have to be exact!
All I had was the Peterson, but he had something more, and after cross checking references, noting the fact that chimney swifts were definitely heard overhead at the same time this dead bird was discovered, Dr Hall broke the bad news to me. This was a Chimney swift. Yes, it had a whiter throat than normal, but it was still within acceptable variation. Also, the odds of only one particular kind of swift out of a whole flock of another kind of swift, striking a window and turning up dead were high.
Id redacted, the bird was still dead.
I still look for Vaux's swifts to fly by every now and again though.