Session["FollowUp"] = "true";

Code on a second page:

Session["FollowUp"] = 1;

Code on a third page:

Session["FollowUp"] = true;

When the bug manifests there will be an InvalidCastException thrown.

Usually this bug will NOT be picked up in testing. It is only when a user does some particular combination of page navigation (or opening a new browser window) that the bug manifests.

What Can We Do?
The First Quick Fix
The first and most simple thing we can do is make sure we never use string literals for session keys. Always use constants and so avoid simple typing mistakes.

private const string limit = "limit";...
Session[limit] = 27;...
Session[limit] = 32;

However, when constants are defined locally (e.g. at page level) we might still re-use the same key unintentionally.

A Better Quick Fix
Rather than define constants on each page, group all session key constants into a single location and provide documentation that will appear in Intellisense. The documentation should clearly indicate what the session variable is used for. For example, define a class just for the session keys:

public static class SessionKeys
{
    /// <summary>
    ///     The maximum ...
    /// </summary>
    public const string Limit = "limit";
}

...

    Session[SessionKeys.Limit] = 27;

When you need a new session variable, if you choose a name that has already been used you will know this when you add the constant to the SessionKeys class. You can see how it is currently being u

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