Whether that was because of a mistake or a characteristic of the system remains to be seen.
I wouldn't say it a characteristic of the system, because, contrary to what some ppl here try to say, it could as easily happen outside US - it's not illegal to do something like that in "ICAO world". Of course, in the US it happens more often due to volume of traffic, but if I have a VFR traffic that wants to cross, it's exactly what I would apply. So it's not the case how the system is designed, but how you play out the given scenario (by you I mean both ATC and pilots).