Yes, I did get a popup error box telling me that I was logged in within another window and had to refresh, but it DID give me the site. This sounds like an issue that may be unrelated or it may be vaguely related to mine. A quick refresh fixed it for me and I haven't gotten it since. What really stood out to me was the HTML code showing in that error box that came up.
For those who are getting it, can you give a little more information? Were you on the site clicking on a thread when it happened? Were you on another site and coming here? Did a refresh work? The more information you guys give for it, the easier the admins can pinpoint the problem.
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And if you can include a cut and paste of the actual error message itself, that would possibly help if this is an unknown issue.