Mario accidentally bumps or does something wrong when filming. This particular problem is very common and had so much film of it that Mario was forced to eat it.
Things don't go as planned, they sometimes get messed up. Examples include landing into Dire Dire Docks with no water with Mario landing into an "airhole" (the whirlpool at the bottom), Mario long jumping off the roof to find there's invisible water around the castle, the water in Wet-Dry World to be much higher than even at the tallest painting jump, and text scrolling along the bottom of the screen spelling "codes by Yoshielectron" going back and forth with Mario commenting.
Mario and the other characters forget their lines sometimes. Some examples shown are Mario talking with some forgetful Toads, Mario walking away from a Boo chasing him who keeps forgetting its line, and Mario forgetting the answer to a joke he tells Toad.
Mario's stupidity makes it really hard to film. Examples shown are Mario trying to kiss a window of Peach, Mario getting hit by a Boo, and Mario wanting to keep the dangerous piano as a pet.
Mario keeps goofing off, he likes to dance randomly in places and go behind people and tease them, and he also likes harassing random people in the castle. An example shown is Mario in a very long sequence of Bob-omb Battlefield.
At the end of the video, Lakitu mentions that despite all this it can be very fun because he gets paid for it. Mario comes over to him and says Toad is having a seizure and he finds it cool. Mario then picks Lakitu up, as they both say goodbye.