Borderlands 2
Moxxi's Tip Jar

After continuously tipping Moxxi will first give you the Bad Touch SMG and than the Good Touch SMG.
The amount of money you tip seems irrelevant.

Regularly you can obtain each of them only once per playthrough.
But you can perform a glitch to acquire multiple Bad/Good Touches:

1. Tip her until she offers you the Weapon.
2. Go to the fast Travel.
3. Travel to a different Location.
4. Travel back to Sanctuary.
5. Pick Up the Weapon. (It will float in the air)

You can repeat this until you have your desired weapon parts.