Consider the below example of a hotel reservation site where the only guidance given is "mistake, month, back". The user is on their own to figure out this one.
The below message from iTunes says that there is not enough room to include all the desired playlists, but the user is not given any hint of what to do or how they could have prevented this. It would be nice to have a view into what is taking all the space or tactics to ensure that there will be enough space on the device. Newer versions of iTunes give suggested solutions to the user.
If a user specifies an unreasonable set of constraints, one of the most powerful things you can do is to provide suggestions to relax the constraints that makes it more workable.
Consider the search suggestion capability that is built into the Google Search Appliance that compensates for user's spelling weaknesses.
Identifying possible misteps of the user and warning them can be beneficial.
Consider the example below of how Firefox warns users of sites that it believes may be malicious.
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