FileUpload control causes 404 error

This can occur when the size of the file being uploaded is outside the proscribed limits.
From The MSDN Article:
The default size limit is 4096 KB (4 MB). You can allow larger files to be uploaded by setting the maxRequestLength attribute of the httpRuntime element. To increase the maximum allowable file size for the entire application, set the maxRequestLength attribute in the Web.config file. To increase the maximum allowable file size for a specified page, set the maxRequestLength attribute inside the location element in Web.config.

Access the return value of an ObjectDataSource Insert method

Handle the OnInserted event of the ObjectDataSource and use the ObjectDataSourceStatusEventArgs.ReturnValue property

Session state

I set my session state timeout to 1 minute, but after a minute has elapsed without interacting with the site, I can still browse to other pages without having to log in again. What is happening?

There is another timeout that you should set in the web.config file:
<forms timeout="1"/>

There is also an issue related to IIS where processes are recycled after being idle for a set period of time. This article explains more