In the Total Cache under the "page cache", I have the following selections: (a) cache post page; (b) cache feeds; (c) cache request only for www site address; (d) don't cache pages for logged in users.
Since I found this problem I turned off "Browser Cache" which probably caused all problems.
To make some test, I temporarily turned on "browser cache" just a minute ago. In the "browser cache" I have a default setup from W3 plugin. Under "global browser cache policy", I have selected:
(a) set last-modified header
(b) set expires header
(c) set cache control header
(d) set W3 total cache header
(e) enable HTTP (gzip) compression
(f) prevent caching of objects after setting change
You can do a following test now:
(a) send me some test message and enter CAPCHA -- all will be fine for the first time;
(b) send a second message and you will get an error since previous CAPCHA is now cached on your browser.