So I started working my way backwards in time. Reading a few days worth of posting at a time. Got back 6 months or so and then all it took was one accidental press on "mark all threads" vs. "mark this page of threads" and BOOM I lost it all.

It would be really nice to have that nuclear option a bit harder to enable - like "are you REALLY REALLY sure you want to mark ALL of these awesome discussions as read?"

There's a "Confirm before mark read" choice in the user control panel that sounds like what you're describing.

Yes but it asked the same thing for page too and they look almost exactly the same so I didn't realize it until 0.2 seconds too late.

An undo would work but it wasn't possible