I have an 8D8 with two right arms (so one looks backwards)- Chris sent me it for free for my focus (many thanks!) and the first thing my brother in law said was "surely these things can be faked" lol
Errors are out there in good numbers, the most sought after error figures are the ones missing paint apps but you have to be careful these aren't stripped. I had one MOC that I sold and I got a premium and more than I paid for it but not insane money (20 quid more than I paid I think)
Some people pay a bit more for errors than others but the amount of people that find them interesting or want one for a focus isn't huge.