Frequently Asked Questions:

Problem Spots:

Creating the Random object repeatedly.
If this is created in the innermost loop (where they generate the next (x,y) point), then they'll end up getting non-random numbers, since each object will be initialized with the current clock time (in milliseconds), and their computer is fast enough to repeatedly create multiple Random objects within the same millisecond
File Hand-In Problem with Fireworks File in Subdirectory
I've been using SourceGear.com's Vault product, which is a full-fledged source-code control product.  Students hand in their homework by checkin in their assignment, and sometimes they forget to check in the file(s) in the Projectile subdirectory.  You should watch out for this as well - it may be easy to grab all the files, but miss the files in the subdirectory.
More to be added, in time