This thread reminds me of a recent incident:

I was ticketed IAD-LGA-BOS using US metal for LGA-BOS (with a UA flight number). Several reservations agents had assured me that standby for IAD-BOS non-stops on UA would be allowed. Mr. Chicken refuesed to allow it, the IAD-BOS gate agent refused to allow it, and the special services desk eventually allowed it but was skeptical.

I thought that standing by for fewer segments was always permissible, even when non-stop itineraries are priced higher. Has there been some change? Were the IAD agents just confused?