In Windows
There is an issue with some Unity assets and path. If in your url or system path
example C:/MyGame/sp ë tialPath/myUnityGame.exe
there is a character as for example " ë " instead of simple “e” then there is a chance to get problems with Unity assets from asset store.
If you try to save the scene it will save it but Unity will think it did not and the “" will persist there on top of the window at the end of the path. Also when you press play the scene was not save "” pops up. Or when you open a new scene there is the need to save it.
Solutions:
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In Hierarchy select all your Game Objects, delate them and make ctrl Z to Undo. Then save or save as and look if the problem persist.
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Look the component that is triggering to save. For example in my case I was using the Unity UI “Content Size Fitter” after you press play then it ask to save. Bug report Info
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If not, try to take out the not latín characters Do not use special characters in the path.
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If that did not work, an extreme long solution that works for me is to compress Assets into a rar, make a new project and decompress the asset and restart Unity multiple times. This means that you need to reset all Unity settings (input, player, etc).