If you alt-tab out of ROBLOX, the tab key and any other key held during the pressing of alt-tab, will remain indefinitely in the response returned by UserInputService:GetPressedKeys().
This makes it unreliable, and any code depending on it (of course) inherently buggy! It’d be great if this was fixed 
Side note: UserInputService:GetPressedKeys() also does not include the most recently pressed key.