Source? Not that I don't believe you, but I'd like to check that out myself.

You will get friendly fire. Not only the poor ghoul you shoot because you think he was a feral, but also during combat.

Remember an random setler lady in FO3, no armor, a baseball bat and fighting an giant radscorpion, she was on the other side of a ravine. I tried to snipe the scorpion but missed and hit the lady who had low health.

Downside I got an serious karma hit, benefit I could buy clover

STEALING
Picking pockets of neutral/friendly NPCs
Taking from neutral/friendly houses/camps etc.
Looting from neutral/friendly NPCs that PC murdered
NOT STEALING
Picking pockets of hostile/enemy NPCs
Taking from hostile/enemy houses/camps etc.
Looting from hostile/enemy NPCs that PC defeated
MURDER
Killing any neutral/friendly NPC
NOT MURDER
Killing any hostile/enemy NPC
NEGATIVE REP
Killing NPC in front of ANY witnesses (and leaving witnesses alive)
Stealing from NPC in front of ANY witnesses (and leaving witnesses alive)
NO REP CHANGE
Killing/stealing from NPC and leaving no witnesses
POSITIVE REP
Helping NPC
Killing an NPCs enemy faction (if that NPC [witness] survives)
EXAMPLE
Sole Survivor murders non hostile member of The Rail Road and loots his corpse in front of a member of The Institute. SS receives negative karma for committing murder and theft, his/her reputation with The Rail Road is lowered because The Institute will talk of this. SS receives a boost to his/her reputation with The Institute for murdering their enemy in front of witnesses who will report this act.