If you place a mine, claymore or c4 and an enemy uses it to kill ppl of your team, make a pop up for the player placing the mine that he killed friendlies. So many ppl do not notice that friendly mines are dangerous.
Just had a match where almost 1/4 of the deaths could have been prevented if ppl stopped placing mines.
and if someone gets too many friendly kills that way prevent them from using mines

. The danger of something like this is, you actually create an incentive to exploit it further. I don't think everyone would react that way, but you would absolutely have some creatures with a refreshed sense of "how can i fuck my team up the most" if you give them a quantifiable value.