Use fuses.
On everything.
Ouch! Yes, fuses are a must with high amps …
Wow, that’s pretty terrifying! Out of interest how many amps caused this?
It is (or rather, “it was”) a 12V 5A power brick.
I suspect I was drawing 4A+, and had been without trouble for a year or so before this happened.
So I suppose the other lesson here is “inspect your installations periodically”.
Cheap Chinese clone perhaps? Let out the magic smoke a little too early … or the timer ran out. 
“Derate! Derate! Derate!”
(Quoted from my new favorite web page: http://wiki.orbswarm.com/index.php?title=Building_Electronics_and_Robots_for_the_Desert )
LOL! Yes, that’s important. Still, if it claims 5A, it should be able to handle 4A fine, perhaps just get warm but to fail like that tells me it wasn’t tested/rated properly.
See also: “How to convert Marketing Watts into Electrical Watts”
https://plus.google.com/112916219338292742137/posts/AWZiH1JbiEJ
/me unplugs the LED array under the sofa until further testing can commence.
Maybe just the physical connection of the plug that failed and started bigger and bigger sparks.
Have you opened up the case? How does it look on the inside?
No forensics yet. I’ll let you know.
(“Up next on CSI: Arduino!”)
Jesus. Maybe the 125W buck converters I got for what looks now like is going to be 30W of draw aren’t such overkill after all. Or fuck it, maybe I should go to 60W 
As those IEC connectors are rated to 10A @ 240VAC, that’s well over what you were using it for.
Most common I have seen is some contaminate gets in the connection and it over heats further to a cascade failure - as you have seen. This contamination can be caused by the lead/unit themselves as well as natural foreign matter. I have even seen eastern PSUs with aluminium pins!
This is why in the UK we have a fuse at the outlet plug to reduce just this sort of thing As well as the distribution board. This plug fuse is to protect the lead NOT the appliance as many people think.
i think this was a problem with a too high contact resistance in the plug. nothing a fuse would have helped.
Looks like arcing in the connector, not a PSU failure. I had a client who lost a computer room due to a similar failure (quite a few years ago now).