ground up by ground rules

[refjef40]

I worked a var. tournament on a field where I was not familer with the ground rules.It was a j.v. field with no fence except in right the varsity field fence was there.I told the coaches during the pre-game the outfield was all open.What I failed to realize was the area between o.b. and the var. fence was open and since is part of the varsity field its normally o.b.,this I found out later!Of course with tying and go ahead runs on base a girl hits one that rolls through that gap and I let it go for a home run.Coach comes out wanting a two base hit saying that is always o.b.My partner says he is right but now reminds me of my pre-game.I let the home run stand but told both coaches that was now o.b. which got the loser fans screaming I changed the rules right then to job them.My question is should have changed the ground rules?