Each time the itemCheckingOut eventReceiver is fired, the following error message is raised:

"Attempted to read or write protected memory. This is often an indication that other memory is corrupt. "

No known workaround at this time, to my knowledge.

Description:

When attempting to cancel an item checkout in the ItemCheckingOut event handler the error "Attempted to read or write protected memory. This is often an indication that other memory is corrupt." is shown to the user. This is a known issue with WSS 3.0 however, there is a workaround. You should set the Status property to SPEventReceiverStatus.CancelNoError instead of SPEventReceiverStatus.CancelWithError or setting properties.Cancel = true;

C# Version

public override void ItemCheckingOut(SPItemEventProperites properties)
{
     // Logic to decide checkout status here 
     properties.Status = SPEventReceiverStatus.CancelNoError;
}

VB Version

Public Overrides Sub ItemCheckingOut(ByVal properties as SPItemEventProperties)
     properties.Status = SPEventReceiverStatus.CancelNoError
End Sub