Somehow vim remembers that I did vimdiff on certain files and keeps opening them in the vimdiff mode. What that means is that when I open that same file again I get a grey bar displayed on the left (like the foldcolumn) but worse when I try to add another file with :sp anotherfile.txt it comes up in a full on diff mode - interlocked, diff highlighted, etc.
It's completely as if I opened the first file with vimdiff somefile.txt, yet I didn't. I opened it with vim somefile.txt and it still behaves like vimdiff, only because in the past I opened it with vimdiff. It's very annoying.
Why does it happen?