Quite big, quite complex, web application.
We were part of a bigger team on this one. Since the client wanted it to be in Java, which we only code for Android, we were in charge not of the backend, but frontend and scripting (off course, on the backend the site involved more technologies that we're not listing).
There were a lot of challenges on this project, both on the back and front end, and they could only be resolved by a full communication and good predisposition of the whole team.