Page runs when it shouldn't

I'm working on a site in C# in VS2010 on a new machine. If I write this:

<asp:DropDownList ID="ddlTest" runat="server" OnSelectedIndexChanged="ddlTest_SelectedIndexChanged" />

and, in the code behind, there is no ddlTest_SelectedIndexChanged function - it should not run. I usually get an error message that says - 'the page behind does not contain a function called "ddlTest_SelectedIndexChanged".

But in this project it runs and I don't get an error message until I go to the page in the browser.

For the first time I created a Web Application - I have always created Web Sites before - do things work differently here?