Microsoft

Managers and mentors do make themselves available to interns.

There is also not much pressure placed on the intern project, which is good and bad. It's good because it allows you room for exploration and a stress-free experience, but the lack of expectations can also be demotivating.

Sidenote: Microsoft really pampers interns with activities, gifts, etc. to show off the best of the Puget Sound.

Constant internal restructuring. Teams and projects may dissolve, and as a result, your work may not be used. (Maybe I was just on a volatile team).

Also, there is no means to find out if other teams are doing similar work as yours or have made headway on the same problems. I would attribute this to scarcity of information and also organizational complexity.