I bricked one of my PSPs (my 3rd, dodgy 2nd-hand one) downgrading from 2.71 to 1.50. I started the downdate and 20 seconds in, the memory stick light stopped and 10 seconds later the screen swiched off. I turned the PSP on again and found I landed up with a brick.
I think what happened was because I forgot to use the one with the bugfix, because that one didn't ruin my other units.