I've noticed that I'm having problems with clicking on common things such as login, looking at the comment list (top left). Up and down votes. Pasting hyperlinks etc.
This has only started happening since Tuesday.
I have this happening on 2 different PCs one using IE at work, and one at home where I use chrome.
The one at home as Ad block but it's disabled for SE sites, while the one at work has nothing of the sort.
Just noticed that I don't get auto complete on the tags. But when I clicked on 'Popular Tags' I was able to select one and now it's back.