See attachment. What could this be? I can't reproduce it and nobody else has mentioned it. He said he got it on 3 other PCs. He said something about it happening on a subscription thread. I'm trying to get more info from him, along with a link to where it was happening:
If he's using IE for it, it may be an add-on he has enabled. Have him try with something like Chrome and if the error pops up to use the Inspect Element to see what the code is. There should be a red flag on the screen when he opens it (Inspect Element) up and he can give you that info.