It was a huge problem and not just for the fighter bombers.
Short bombing by heavy and medium bombers also occurred. It was described as severely affecting morale and eroding confidence in the air force.
For example during the second phase of "Totalise" two formations of B-17s straddled Canadian and Polish troops who were in the open, ready to mount an attack, with nearly 1,000 90lb fragmentation bombs. When allied guns fired on the errant bombers troops on the ground cheered!
Cheers
Steve