The bootloader is signed, and if you flash a modified bootloader(for instance using a recovery) it see that it has been modified and that the signature doesn't match and stop during boot as soon as it finds the modified bootloader(so it won't try other boot sources such as usb, or other devices).
I did exactly that(I tought that it would try other boot methods instead because a user reported that it did that when his NAND was damanged) and I bricked my device.
In order to recovery I removed the xOM5 resistor and didn't do anything else hardware related, then I remounted my phone and used hummingbird-hibl to resurect my phone, this time it worked(with the resistor removed).
The issue with it¶
According to Rebellos on older revisions of the s5pc110 such thing(removing the resistor and not putting it back) can cause issues such as hanging the CPU.(it's unknown if newer revisions have a protection against that...).