We've heard some stories about destination mistakes made by travel agents. But it was more like same city, different country (like Paris-TX instead of Paris-France), and not IATA code mistakes.
Baggage tag mistakes should be rare now, as they are printed from PNR.

Otherwise, other mistakes concern date/time.
For example : forgetting to add one day for a flight departing 00:10, or forgetting to subtract one day (departure) for a flight arriving +1.