That almost happened with me today.
I didn’t know that once I enabled this mode, and happen to close that incognito window tab, there wasn’t anyway to log back in!!!
Holy God!!!
How easy it was to lock my self out!!!
And its not less than miracle that just within 5 minutes (of disabling the 'read-only mode) I stumbled upon this post.!!!
Now at least I’ll try to remember this thing in future, whenever I want to tinker some admin settings!!!