I don't know if there are other ways to end up in this situation, but I have a large number of OFX imported transactions, where the memo is put in all three splits (investment account, the security, and the category).  If I edit the transaction in the investment account and empty the memo, it actually is not removed from the category split.  I discovered this by looking at a category based report.  It is possible to edit/clear the memo by editing the transaction in the brokerage account, where the category shows up as a split transaction.

I'd love to see an easier way of getting to this.  If I edit the transaction in the investment account itself, the category split is not shown.  I can see the memo looking the the Category View, but editing transactions in that context is not allowed.

This is a wishlist, because I'm not even certain of the best way to approach this.  It might partly be done by notifying the user any time a memo is edited, where it will not make the same edit in all splits of the transaction.