Historically it's extremely common but mostly this is tanks and aircraft. Most losses in Gulf War w

It was uncommon in Iraq/Afghanistan. Usually it was say, Marines shooting Soldiers or vise versa because they did'nt communicate on the same net and were not aware of other's positions and usually at night. Blue on Green and vise versa was common because they rarely if ever communicated or coordinated on anything that they were doing. They see armed people sneaking around can't confirm who they are and so they open fire.

The solution to this is the BFT2 or whatever its called now. Usually friendly fire is because units don't know the relative positions of friendlies and enemies or they get disoriented.