when you load a saved game in rct... the game automaticaly checks for things like too much flat land, or not enough trees ect. it's a type of cheat protection. you need to download the drexler patch, which stops the game from checking those things.
visit my site below, go to the cheats and trainers section, and download the version of the drexler patch that is right for whatever version you have.
also, you cannot load a saved game created with the corkscrew follies or loopy landscapes expansion unless you have them installed.
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www.angelfire.com/in3/rct