Red triangle on element?

(This post is a question. It's created here with the purpose of being able to link to it from certain web discussions. But, of course, if you happen to know the answer, you are very welcome to present the answer here)

What does this red triangle mean? - and in how do I remove it?

I don't know how I happened to generate this red triangle.

If you happen to know, please let me know how to remove it.