One time it cost me an extra $50 to do DCA - DFW - LIM - MIA - ORD - DCA instead of just DCA - ORD - DCA when booking last minute ($500 vs. $550). I had to be in ORD for something and was consulting at the time so I happily paid the extra $50 and just worked from the hotel in Lima on Friday (and I was an EXP so the SWU to the old 763 J seats helped make it doable).
I think when I looked at the fare rules it was because DCA - LIM allowed a free stopover on the way home (which I used at ORD since that's where I actually needed to be) and the DCA - ORD fare was only available in expensive fare buckets.