Mixed metal itineraries wreak havoc on my flights if I get a UDU on United metal. I have no idea why.
I accrue to OnePass, and the miles don't post to OnePass for weeks. I've had to call in for a number of different PNRs where I am listed as a no-show for the segment in question.
Sticking to only CO or UA metal on a given PNR (and avoiding booking UA flights on CO ticket stock and vice versa) would avoid this, but I need the convenience of scheduling mixed metal itineraries; the combined route network is the main reason I fly UA/CO.