I think flashing the wrong kernel or modem is most dangerous. Can a ROM do harm? As long as you flash proper kernel before booting system?
Flashing a modem in cwm is never dangerous. Flashing a rom can be an issue if something in it is not compatible. Same for a kernel.
We can usually flash a i9100 kernel and usually we just lose our buttons properly. We can flash a i9100 modem and just lose the ability for making calls. Flashing a rom can put you in a boot loop as well. Now USUALLY if you know what your doing to any extent all the above can be fixed by flashing the CORRECT kernel, modem, rom for our phone.
Where we get in trouble is if we flash sometjing that changes either of the bootloaders or flashing a rom, kernel from a device NOT EVEN CLOSE to our phone. For example all the Skyrocket people flashing something from our forum onto their phone. Or flashing or trying to flash something with rom manager.
Sent from My SGH-I777 Running Spiderman Themed AOKP or Spiderman Themed CM10