OpenTTD is an open source simulation game based upon Transport Tycoon Deluxe

In the original Transport Tycoon (but not OpenTTD), if you would build a tunnel from one end of the map to the other, the amout of money to pay gets so large that it triggers a (signed) integer overflow. So you can start your campaign with billions of dollars!

Windows Solitaire had this two. If you kept dealing without playing, once you'd spent over $32,768, you'd instantly be in the positive nearly that much.