The computer asked me to review this question. When I tried to enter a standard show your work comment I got a message that it was a read-only question and I could not submit a comment. I opened the question in another tab and was able to enter my comment there. I have never experienced this behavior before. Perhaps it is a bug, perhaps it is a feature.
Neither a bug nor a feature: for a couple of minutes all Stack Exchange sites were in read-only mode, with a black banner on top of page proclaiming exactly that. (I only know about meta.SO and Math for sure, but I assume this was a network-wide maintainance thing.) Back to normal process of copy-pasting comments around the site.