Thanks for all of the fast replies. I'm using Ilford multi grade developer. This was a brand new bottle I opened last night.
Another member pm'd me saying maybe I got some stop bath in the developer. I suppose it is possible, though I was being very careful.
They were different negatives, but the problem came up between two different shots on the same roll of FP5+. I noticed the same highly increased exposure when I switched to a negative on Acros.
I'm going to give it another go tonight. I'm going to mix a new batch of developer and go from there. Maybe spend some more time on one negative.