Some years ago I yelled "On your left" at a pair of riders who where all over the place on the local bike trail.

The male of the pair then proceeded to follow me for about 3 miles until I stopped for a left turn when he jumped off his bike and blindsided me with a few punches.

Several bystanders later and an a couple of cops he claimed I had called him some name or other, which prompted the cop to say "Well, I guess he was right."

Moral of the story: I don't alert people anymore. Not worth it.