yeah so many guitarists are just gear-tarded.
I've loaned out some of my stuff when I put on gigs for guys in bands to make it easier for them to travel- usually my 2 guitar cabs and they bring their own heads.
The amount of times i've seen dudes turn on their amp, leave it on and then plug in the speaker lead, had one guy with a dual rec who couldn't figure out why he wasn't getting any sound- turns out he didn't have the speaker cable in, his amp sounded very strange after it, I'm pretty convinced he did some serious damage. Found out later on that night that particular guitarist is actually a sound engineer too! he's no idea that what he did was wrong, he laughed about it!
Also- line 6 have some heads with retarded outputs, 2 8 ohm outs and thats it, no switchers or other impedance options, i can't figure why they have it like that, I've seen not 1 but 2 guys plug both the 8 ohm outs into the back of my laney 4 X12 which is 16 ohm, they plug 1 into the input and the other into the link out! I know you can use a different impedance into it and suffer a power loss but if you get it the wrong way around your gonna fry the lot.
I loaned my marshall 2X12 which is 8 ohm to a friend earlier on in the week with an amp that another friend built for a song he's recording for college that i'm playing on. when i came in they had it set up with the wrong impedance even though its clearly marked on the back, when i said it to him he said it says 16, even though i've owned the thing for years and use it on a weekly basis he tried to tell me he was right, obviously another quick look at the back of the amp proved me right.
Also worth mentioning that I've only seen about 2 guys using actual speaker cable, everyone else just uses guitar leads!
It's for these reasons that I have the stereo in on my marshall cab taped over so people wont plug into it by accident or try what the OP did.