value session_data field in ci_sessions table is NULL - Why?

[eluser]moonbeetle[/eluser]
What I'm trying to figure out is why my user authentication seems to fail for some users.
For me it works great, every time I login. But for users Bob and Jeff it doesn't. Bob and Jeff both work at he same company and their internet access is most likely monitored. They both have their own PC.

When user Bob is authenticated he should get a greeting saying "Hello Bob,". Most of the time this works but sometimes he gets a greeting "Hello Jeff,". So I'm trying to figure out if it's a caching problem, a bug with the NG Session library, a problem with cookies not being stored properly or just me having written bad code.