Make an application that, whenever the left mouse button was clicked, it would open a dialog asking if the user really wants to close the top window if activated. Every time they would click it would say "Are you sure you want to close Microsoft Word?", in Word, for example.

To all new devs:
- Your language of choice is fine.
- There is no superior way to indent, yours is fine.
- Your IDE is fine.
- Your OS is fine.

Unless you work in my team, of course.

The coolest thing I have worked on is a League of Legends sandbox. A reverse engineering of the official server, so we could do whatever we want.