A recent web application that I worked on had some strange issues within a day of going live.
The configuration has 2 nodes serving web pages over http and https behind a load balancer.
Secure pages and logins are served over https, all other URLs are directed to be served over http.
Some users reported that they could successfully login, but the next link they clicked would lose the login.
It turned out that our load balancer configuration had separate stickiness tables for http and https, so there was some chance of a user being connected to node1 for http, and node2 for https. In some situations that would not matter, but the underlying application does not share session state across the cluster.