So I get to this aeration job I have scheduled for this evening, and the home owner tells me he can't find his sprinkler heads. He's owned the house for just under a year, and never used the system. He scoured the lawn and could only find one head. So I tell him that if I hit a head, I'm not responsible since they aren't marked. He said fine, no biggie, he's not planning on using the system anyway. And then I proceeded to hit a half dozen heads all over the yard. They were mostly buried all throughout the yard, covered with dead grass and overgrown. They were the big black ones, where the unit pops up and circulates over an area. A couple of them I just hit the covers, but I ruined a couple more. He was not happy. I explained that I warned him ahead of time and he agreed and paid me. But I still feel bad. The yard was too big for me to walk ahead of time, and I could have never seen them the way they were obstructed. Earlier in the day I hit a similar head at a mowing customer's yard, but the tine just bounced off it no damage. I gave the customer a bundle of flags to mark the heads, she missed it. What a day.