lol.. you are just experiencing how smart border collie can be, they were originally hunting dogs, so in the course of time they always develop a trick or two to get what they want since they don't hunt much now..
huh? border collies hunting dogs? first time I heard that... They were bred for herding. But yes, they're smart.
But this is really not an indication of a dog being smart... it's essentially the basic dog training (and, as you pointed out, a mistake made by OP). You have an active breed that loves being outside and every time he/she sits by the door, you take them outside to potty- but also go for a walk, play with them. Well, then they simply associate sitting by the door with going outside... and not with going outside to potty.