About Dire Straits, besides the MFN intro idea, who wrote the intro, etc, there are songs that we know must had more than contributions.
For example, we know TR was developed by MK and Alan, hand by hand, during the soundchecks of the On Location tour, it's MK song but, where Alan's contributions ends as such and develope into co-composing? A grey line.
Also, we know that Paul Franklin was playing some steel guitar warm up licks, Jeff Porcaro followed him, and MK liked so much that he included that in "Calling Elvis". Does it means that Franklin and Porcaro deserve credits for creating one of the main rythmic parts of the song? Or it was just a contribution?
I'm sure that there are lot of more examples, as Quizzy mentioned the Richard Bennett intro for "Rüdiger".