I've had this problem off-and-on for awhile. If I am composing a message, go off to another tab and copy its URL, then attempt to paste the resulting text using Ctrl-V, instead of pasting the text, Gmail often sends the message. Pasting using the context menu works (right mouse, select "Paste"). I don't recall seeing this happen with other text. Or should I say, text copied from anywhere other than the URL field.
I've tried searching for this problem, but the number of common words in my searches seems to thwart my attempts at finding out if others have experienced this problem. I mostly get hits for stuff like, "how do I paste text into Gmail?"
For the purpose of the current discussion, assume I am using Gmail in Chrome 49.0.2623.87 on openSuSE 12.2 (both obsolete, I realize, but there's nothing I can do about that), though this has occurred to me on my Mac running a newer version of Chrome and up-to-date version of Mac OS X.