At home, we bought a good looking flash, the YongNuo YN468 that worked flawlessly... for about three weeks.
Then it started firing once every 5 or 8 trigger commands, no matter if they are sent by the camera, the remote receiver or the test button on the rear panel of the flash.
After a few weeks resting in a drawer, the internet started to report the same failure on other flash units time and again.
Somebody found the problem: a small capacitor in the flash head. Later, he reported that the flash worked fine after the replacement.
Other people replaced the capacitor and reported success, so I decided to take the plunge.
End result: it is working fine now, and I'm sure the original capacitor was open.