Can you explain a bit more?
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One of the most documented cases is a CX A330 with P&W engines who had electronic problems on both engines after tanking contaminated fuel.
It actually happens all the time. Each time something is changed to the basic off the shelf model, seats require new certification (can be 'minor' if it's just a special new fabric used or something or it can be extensive if something more structural is customized). And plenty of airlines order customized models of existing models (also SN). And in addition the cabin as a whole also needs to be certified again depending on what has been changed.