How to troubleshoot possible intermittent issues with cua-mode in recent Emacs?

[N. Jackson]

In recent days (probably for less than a week, but perhaps for two or
three weeks) I've been experiencing a possible intermittent problem with
CUA Mode.

I'm looking for suggestions about for what to look for next time the
problem occurs.

I am running Emacs 25.0.95 from the prerelease tarball. I might have
seen this problem (if it really exists) with 25.0.94 too but I don't
think so. I certainly didn't see it before 25.0.94.

The symptoms are that I select some text (using the shifted arrow
keys), copy it with C-c, switch to another application (typically an
input form in web browser) and paste. At this point I realise that my
copy operation failed, as I get a stale clipboard value pasted.
Suspecting user error, I switch back to Emacs and try my copy again and
see in the minibuffer something like "C-c C-c is undefined".

That is, the initial C-c seems to have been treated as a prefix key not
as a copy command.

Once I get into this state, C-c no longer works for copy. Looking at the
Options menu, "Use CUA Keys" is still checked. Unchecking it, then
checking it again does not seem to help to get C-c behaving again.

However, doing a copy with M-w seems to clear the problem (until it
comes back a few days later).

I apologise that some of this information is a bit tentative. I tend to
use copy and paste fully on autopilot.

I've been using CUA Mode in Emacs for years, so I don't think this is
user error. (But of course it might be.)

Note: I have cua-prefix-override-inhibit-delay set to 0.3 and I'm half
wondering if something is going wrong with this timer.

Troubleshooting suggestions welcome. Thanks.

Regards,
N.