When Marshall comes out of his room with the book about the Rosswell Conspiracy, he's holding the cover and first pages bent under the book, next shot he's just holding them normally, then they're bent again.
Ted claims that being a 9.6 match is 11.45% better than an 8.5 match. In fact, a 9.6 is 12.94% greater than an 8.5; an 8.5 is, however, 11.45% WORSE than a 9.6.
After Ted returns from his appointment with the dermatologist, he tells everyone that she's engaged. A few minutes later, Lily is trying to talk him out of interfering with the wedding plans by pursuing her. She says that the doctor is getting married on Saturday. But Lily doesn't know this information. The audience knows because we heard it from the doctor at the appointment. But Ted never mentioned when the wedding was to take place and no time has passed for Lily to have learned that specific information off-screen yet. However, everything the audience sees involving Present-Day Ted is actually being told in flashback by Future-Ted. He may be simplifying or truncating the story, or remembering it incorrectly.
Ted tells Sarah he was confused at Love Solutions because of the math Ellen used, though all math involved was relatively basic. Considering his career as an Architect, which requires one to be proficient in advanced math, he should have been able to follow fairly easily.
When everyone is watching Robin's report in the bar at the beginning of the scene, she signs out, stating her full name. However, Robin mispronounces her last name. She pronounces it phonetically, saying "BAT-ski" instead of "BOT-ski."
Ted says his guilty pleasure song, "Summer Breeze" is by "Seals and Croft" instead of "Seals and Crofts".
When Ted asked Sarah to go out on Saturday she said she is getting married that day, however she is not wearing an engagement ring.