It would be fantastic if admins could log in "as" any member they want, or even better, could we have a permission for "Can assume other members' identities"?
Any member of a group with that permission set, would have the ability to "Log on as..." and then pick a member, and they would then see whatever that user sees when they log in, until they click "Revert to (username)".
Obviously this would be used only by admins, to enable them to fix issues, and test permissions.
There should also be a corresponding permission "Do not allow this account's identity to be assumed by members of the "Can assume other members' identities"" group.
I've used this feature in countless environments and it works very well.
If someone wanted to write this for me, I'd be willing to pay for it.