How reliable is vent sexing?

[PepeDeSilva]

I did my first big cull last week, butchered 6 of my 9 birds trying to get all but one of the roosters. I used the vent sexing technique since all my quail are either white or tuxedo. All 6 birds had the foam come out when I pressed on the vent, BUT when I was butchering I discovered one was actually a hen.
Is this a pretty common occurrence when vent sexing?

 

[DK newbie]

If the hen has been mated shortly before you vent sex it, it can have some foam, though it shouldn't be as much as a roo.