I actually don't know how to reproduce this bug. In a nutshell, I tried posting a question and answer (together) on Super User. Every time I hit the submit button, the page would reload, with all my content intact. No question was posted, and no error message was showing.
The details can be be found on this Meta SU post. The bottom line is: although the answer for why it probably didn't work was found (connecting from a location with multiple users behind one IP address), I still should have received an error message, and I received none.
Happened on Chrome OS, version 62.0.3202.74.