Eevee showed up as Magikarp?
Just now, a CP 50-something Magikarp appeared. After catching it, though, it said I caught Eevee, and it is now in my storage as an Eevee. Unfortunately of before catching the Eevee (I wasn't expecting it to be one to begin with), but I do have an image of the Eevee In question. Has anyone seen anything like this before?

Answers
Yes, I have experienced something similar once before. I think it was when Gen 2 came out and I caught three Swinub (I think it was Swinub anyway) and one turned out to be something else (can't remember what exactly).
Never heard about it happening to anyone else though, so it must be some very rare bug.
I actually had the exact same thing happen last night. I had an eevee on my map and ignored it. Checked my phone maybe 5 minutes later and now had a magikarp on my map in the same spot. It was a magikarp on the catch screen, but as soon as I caught it, it awarded eevee candy and became an eevee.
This was not in a nest, but was in my home where I get very few spawns.
Mine wasn't in a nest, either. I was in my house where I can usually get a decent number of spawns, but I've never seen or heard of something like this, so I was very confused
Now that I think of it, when I originally went to catch it, it was acting as if the Magikarp wasn't there at all. All it did Wass press the area on the map underneath the Magikarp...