I've been using, writing about and talking about ASP.NET 2.0's OnClientClick mostly in relation to some of the Ink on the Web work I have been doing. OnClientClick (who's functionality you could achieve in the past with control attributes) allows you to associate client side script AND server side code to the click event of a button.

One thing I hadn't run into and therefore wasn't aware of was the OnClientClick conflicts with another one of ASP.NET 2.0's new features, Cross Page Postbacks. Teemu Keiski explains why the two won't work together in this blog post.