Recently during a performance testing assignment, I came across a system that produced business reports that would accept approximately 80 users and then allow new users still to login but each new user would just wait at the hour glass for ever. While the new existing logged on users could still function and had good response times. All the usual metrics looked good so no immediate issues where identified.
After much searching it was discovered that the desk top heap had been exceeded and the application refused to start any new report processes. Desktop heap is usually used for storing data associated with on screen application, as is commonly seen when you try to open to many application on your desktop. So it was a surprise to see this occur on a server.
The moral of the story check the logs!