User blog comment:Otakon7/Otakon7's Buddyfight Simulator/@comment-28173866-20161120200518/@comment-2234004-20161121181222

It's an old error that happens when either the connection is bad (aka, lag) or in response to a certain action which fails to do what it was intended to do.

The latter can only be logged if you have a console open (in most browsers, it's pressing F12, then switching to the console tab, where it will say what the error is)