two bricked gs3s in one day, where did I mess up in both instances?

i think i bricked two old s3s today waiting on the little usb to come from ebay to see if i can fix them
first brick bootloader by accident to pda instead of bootloader
the second worked well from the older baseband to the mj2 to the mjb to ne4 rom without a reboot loaded, and when i did a reset it shut off and didn't come back on.

Where did I mess up in both instances?