When I tried to suggest an edit to this question, I got an error message telling me that I can't create new tags when I was not.
When I removed the [game-development], which is blacklisted on GameDev, the problem went away.
As I understand it, when the question was migrated from Stack Overflow to GameDev, the question was not stripped of its game-development tag. However, since it's blacklisted, the site ignored it. As far the site was concerned, there were only four tags on the question, not five.
So, when I tried to edit the question, the site believed I was trying to create the [game-development] tag even though it was there since before I came to edit the question.