Seeing errors in the log should not cause great concern if no users are reporting problems. The wild wild web is crawling with mischief that will poke at your site looking for vulnerabilities or trying to cause problems. It is very easy to generate a request that results in an error being logged.
If you see these errors come in with very rapid succession like multiple times per second from the same ip address then it is less likely to be a user with a slow connection and more likely to be a mischievous person or bot. In some cases if I see too much of this kind of thing form a particular ip address I will ban the ip address. If you see it not coming in very fast then it could be a user with a slow connection clicking a button before the page loads completely, but if there are no buttons on the page, then it again seems more likely to be mischief.
In any case this error is causing no harm to your site and is not cause for alarm.