Scratching our horse has a tendency to prompt her to scratch back. We've more or less taught her to just do that with her lip (seriously, horse lips are flexible and strong enough to do that just fine) rather than her teeth like she would with other horses, but sometimes, those habits slip back in...
So she sometimes wonders why we tell her off when she just wants to return the favour. I always feel bad, but I'd rather not have her rip holes in my clothes (nor skin, for that matter).