Vim won't write file without a "!" sometimes (E13)

Question

Very often (as in multiple times an hour), when I save my file :w, vim tells me "E13: File exists (add ! to override)"

I don't know why it does this, I can't reliably reproduce it, it feels random.

This is my vimrc, note that it sets nobackup, nowritebackup, and noswapfile, and there is a function to strip trailing whitespace that gets run when I save a file.

Also, I tend to have 20 vims open at once, all backgrounded, often editing the same file. Also not improbable that I have the same buffer open in multiple windows (ie :vsp) and might open it, then reopen it with the e command a lot, possibly from a relative filepath, or possibly from an absolute one (the cmap %/ <C-R>=expand("%:p:h")."/"<CR>). No idea if any of this matters. Next time I have this issue, I'll check my ls and report anything odd.

Update:

When I tried to save "lib/seeing_is_believing/wrap_expressions.rb" (note that this is a different file than the one in the gif), this happened again. Here is the ss, it's buffer 3:

Update2 (for @mMontu)

I just realized that there are two errors happening here. The one in the screenshot is the readonly thing. The one in the gif is the more common one, E13: File exists (add ! to override)

The one I just hit is E13 File exists, for this one, readonly is not set:

Update 3

I'm pretty sure the problem is the ZoomWin plugin. I had switched it up to a newer version, and it simply didn't work right. So I stopped using it for a bit, and didn't have this issue. Then switched it back, b/c I miss its functionality (it's my favourite vim plugin), and the problems started again. Possibly it's ZoomWin in conjunction with NerdTree window. Probably not the lib authors' faults, vim in general seems fragile and buggy. Maybe I'll try NeoVim, see if they've done a better job. Maybe it's time to try Atom or Emacs again.

Answer 1

It seems that if there were read errors opening the file, Vim will print an error on :w. This can be seen by running :f:

"MANIFEST.in" [Read errors] 1 line --100%--

The errors aren't necessarily errors in reading the contents of file; they can be caused by a plugin.