Comment 3 for bug 157683

So, AFAI can see, there are three situations:

1. CD is debian but non-ubuntu
2. CD is ubuntu, but earlier version
3. CD is ubuntu but later version

Case 1 typically occurs when people are told that ubuntu is based on debian and they assume things lke compatibility.
Case 2 very likely for a hasty userf who wants to fix something broken due to any reason at all, and has no access to the internet or latest copy and wants to "at least restore back to old state". If things don't go well, he breaks synaptic, like I did and it won't allow any further action!!
Case 3 should probably not be a problem, but packages can get deprecated, as well.

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So IMO, a broad warning at the time of pressing the "Add CDROM" button is a must.
Immaterial of which CD is about to be used.
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Any later second and further warnings would depend on the script that identifies the cdrom package version (if it does).