If a user clicks on a checksum link that has expired, I don't think they get any notification to that effect. This can mean, for example, that when renewing a membership, they might create a duplicate record if they use a different email etc.
Could civicrm provide a warning to the user?
I think this idea may be too specific for general use, but an extension that offers a 'self service' checksum option, ie 'please enter your email address and check your emails for a new link' might be useful.
Thoughts?