Other mistake: At the start of the episode it can be seen that the DVD player only has a power cord connected. Shortly afterwards only one cable (power) is seen connected to the TV set, then a few seconds later Ted plays a DVD.
Plot hole: When Robin and Lily find Barney's secret Robin notebook, Barney and Ted are at the class, so Barney should have the notebook with him.
Suggested correction: This is a character mistake at best, and quite a common one. Students constantly forget their textbooks, homework, notes, supplies etc.
It should be noted that the plot hole isn't talking about "students." This isn't about forgetting textbooks, supplies, or homework. Barney is trying to be a better boyfriend to Robin and wants to learn Ted's secrets since Ted had dated Robin. If Barney has been writing everything down before, the mistake is pointing out why isn't he writing it down this time, except to allow the writers to have Lily and Robin find the notebook and the class.
Continuity mistake: Ted says that Marshall and he only ate jerky and drank Tantrum on their road trips. In 2-17 "Arrivederci, Fiero" however it's established that Marshall had a rule forbidding any kind of food or drink, even groceries, in his Fiero. (00:01:15)
Suggested correction: Marshall came up with the rule because of Lily and Robin. Before they met, in the era when Ted and Marshall had road trips, they would've had food and drinks.
Nope, the no food or drinks rule is because his brothers made him get 12 hot cups of coffee without lids and drive with them naked, and he got burnt. That was when he got the car, before he met Ted.
Character mistake: Marshall's whole career focus is saving the environment, so it's a poorly written character choice that he buys plastic glasses to avoid a dish-washing feud, boasting that they can just throw them away. (00:20:10)
Continuity mistake: In the 7th episode of Season 5, the scenes when the "Storm-Trooper" (actually, a robot) passes by The Window do not match. The first time we see it, from the outside, as soon as he passes, Robin notices him and points towards him, but the second time, from the inside, she doesn't point at him.
Continuity mistake: The scenes before Robin and Barney leave the diner vary depending on the angle. From outside we see that Robin fixes Barney's suit before they leave, but from the inside she doesn't.
The Playbook - S5-E8
Continuity mistake: In season 5 episode 8, Marshall and Ted are playing on an Xbox 360 Console. If you look carefully, you see that Ted's Xbox controller has no batteries in it, yet he is playing the game with Marshall, and then a few scenes change and so do the batteries. Scene 1, 12 minutes and 23 seconds: Marshall's remotes has a battery in while Ted's is out. Scene 2, 12m 40s: Marshall's Xbox control has no battery in. Scene 3, 12m 51s: Marshall's battery is back in the controller. Scene 4, 12m 58s: Marshall's battery is out again. Scene 5, 13m 44s: Marshall's battery is back in.
Slapsgiving 2: Revenge of the Slap - S5-E9
Audio problem: When Lily's dad shows up at her house unexpectedly, Lily starts walking to open the door before the bell rings. (00:02:30)
Revealing mistake: In the big song and dance number, during the line "Girls will go and girls will come, but there's only one absolute", one of the dancers in the second line that runs in from the right is wearing trainers, not dress shoes.
Continuity mistake: Lily and Marshall mention that they've been using the same toothbrush ever since they've been together, but in the 14th episode of the 1st season, "Zip, Zip, Zip", Marshall and Lily are simultaneously brushing their teeth with a tooth brush each (before Ted and Victoria get home). (00:05:05)
Suggested correction: Many people buy another toothbrush to take on trips and since they were supposed to be going away, they probably decided to use them at home, as if this was their trip.
Say Cheese - S5-E18
Continuity mistake: Barney claims in this episode he never "takes a bad picture". But in earlier episode "The Sexless Innkeeper" there were plenty of bad pictures of Barney in Marshall's Best Night Ever Montage.
Continuity mistake: When Barney and Ted are calling dibs on the girl at the bar, in the closeup her book cover clearly reads "Brideshead Revisited", but in the wide shot it's something else entirely, spread across several lines.
Subway Wars - S6-E4
Revealing mistake: When Ted uses his laptop to get directions to the restaurant, Bing shows Los Angeles (where it's filmed) as their current location, instead of NY where the show is set. (00:02:15)
Suggested correction: Depending on the location settings on his laptop, they could be giving a false reading. My laptop shows I'm in Cardiff Wales while in fact I'm nowhere near.
The show was actually filmed on a film set in Los Angeles which is probably why the laptop says Los Angeles as the current location. This most probably slipped past the editing crew.
Continuity mistake: There is a lot of foam in the glasses when Barney pours champagne, however after camera shot changes all the foam is gone. (00:10:15)
Continuity mistake: Randy's noise bleed moves from his left nostril to his right and then back again. (00:14:20 - 00:15:00)
Continuity mistake: When Randy reveals that he didn't sleep with Robin, the blood from his nose switches nostrils between shots.
Other mistake: When Marshall brings the large photo of The Captain into the apartment, he claims he took the photo the last time that The Captain picked up Zoey, but the background of the photo is not the inside of a vehicle.
Character mistake: In another episode it is revealed that Robin hates nicknames and reacts badly when she, or anyone else, is given a nickname. However, in this episode her and Marshall give each other nicknames, and she doesn't seem bothered at all.
Other mistake: Robin says she burned the dinner because she mistook the temperature scale on the oven for Celsius. However a given temperature above -40 is always higher in Fahrenheit, so by assuming the oven to be labeled in Celsius she would have undercooked the food. For example: If the directions were to cook at 400°F, she would set it around 200°F (thinking the oven was Celsius.) If the instructions were to cook at 200°C, she would still set the oven to 200°F, undercooking the food in both cases.
