When Ted goes on the date with the girl who has her arms heavily tattooed, there is a point where the girl turns into Robin. When "Robin" bends her elbow you can see fabric bunching up, revealing she is actually just wearing a long sleeved shirt dyed to resemble tattoos.
When Barney is trying to help Ted with online dating, he says he uses his "own, personal logarithms" to search for the right girl for Ted. Given he's setting up search criteria, the line should have been "algorithms".