When Ted is teaching Barney the Robin's erotic zones, first he draw marks on the left silhouette, and at the next scene the marks are on the right silhouette.
Ted teaches Barney that Robin hates nicknames, however, in Season 2, Episode 1 'Where were we', Robin calls Ted 'Teddy bear'
Barney tries to leave Robin's room through the window, although there is not a fire ladder near this window. The ladders are on the next window, frequently used by the group to access to the roof, but the fire balcony extends to both bedroom windows for egress per code at the time of the building construction.
Right after Robin smashes Barney's briefcase with a sledgehammer, she throws the sledgehammer onto the table and the sledgehammer bounces.
The barrel that is shown outside. Is has a darker and different coloring.
Robin is reading Barney's notebook for the "Robin 101" class when she discovers the class was happening right then. If Barney was in the class, he should have had his notebook with him.