Nothing surprises me when it comes to things a parent might find to complain about when it involves their "little darlings". I've heard some doosies!
A few from when I coached...
One league had a rule about the minimum number of innings a player had to play and I was rotating kids in and out the whole game. One kid had started, got in his minimum, then sat out a couple of innings. When I went to put him back in the game, he tells me his knee is sore and he doesn't want to play, so I left him out.
The next day, his dad calls and gets my voicemail. The guy just literally rips into me me with a five minute message about how unfair it was that his kid only played three innings, how he expected that things had better change and how my priorities as a coach must be out of whack because it was obvious that I was more concerned about winning that letting as many kids as possible play.
I called the guy back the first chance I got. I let him know that I got his message and before he had a chance to start in on me again, asked him if he was aware that his son specifically requested not to go back in the game. He wasn't, sputtered some excuse and offered a weak apology. The guy never said one more word to me the rest of the entire season.
Another guy fancied his kid as the the second coming of Johnny Bench and wanted her as our catcher. One game, his daughter caught the first three innings, then played another position. The guy reams me for taking his kid out "too soon".
A few games later, his kid caught five innings. She made a few errors in her fifth inning. After the game, the guy reams me for "leaving her in too long"!
I guess the magic number of inning for this kid was exactly four...
The worst may have been the parent (mom) that had two brothers playing for me. The lady was friendly all year and I never had a problem with her. Her kids were enthusiastic and talented and they were rewarded by playing every inning of every game up until that point of the season. Toward the end of one game, I finally had her sons sit out a couple of innings to get some other players in.
This lady immediately pulled her kids from the bench and just left! She didn't even say a word to any of the coaches. I had to call her the next day to find out what was going on. She was mad that her kids had to sit two innings and informed me that they had quit the team- and they never came back!