If you try to launch a fullscreen game in the Game Player that presumably doesn't have a full screen option. The game fails to launch because the fullscreen property is nil. I found this to be the case with [Minigolfmania] and [Hunt For Food].
Steam Standard Edition
Windows 7 64-bit
Non-beta branch
Log
Launching process "E:\Program Files\Portable\Steam\steamapps\common\Leadwerks Indie Edition\Templates\Lua\$PROJECT_NAME.exe" -sandbox +game "C:\Users\Mike\AppData\Local\Leadwerks\Workshop\251810\Game_311533257_76561198002835201_Minigolfmania.zip" +screenwidth 1920 +screenheight 1200 -fullscreen +lightquality 2 +multisample 4
Initializing Lua...
Lua sandboxing enabled.
Executing file "E:/Program Files/Portable/Steam/steamapps/common/Leadwerks Indie Edition/Templates/Common/Scripts/Error.lua"
Executing file "E:/Program Files/Portable/Steam/steamapps/common/Leadwerks Indie Edition/Templates/Common/Scripts/App.lua"
Lua Error: [string "E:/Program Files/Portable/Steam/steamapps/common/Leadwerks Indie Edition/Templates/Common/Scripts/App.lua"]:26: attempt to perform arithmetic on field 'Fullscreen' (a nil value)
Process complete.