Continuity mistake: When Sam leaves for his job interviews he sees his parents and drops his jacket, he then walks back into the apartment and comes back out with his jacket in his hands.
Continuity mistake: When Optimus arrives in the street where he battles Shockwave, he passes the firetruck (behind which Sideswipe takes cover) twice.
Continuity mistake: When Sam first meets Dylan Gould at his showroom/office, there is a close up of Dylan using an iPad which he hands to his assistant. His assistant then folds her arms, covering the Apple logo. In the next shot it has been repositioned to show the logo.
Continuity mistake: When Ironhide crashes into the remaining dreads and transforms, one of the dreads is shown falling into some kind of wooden box covered with glass breaking it apart. In the next shot the Decepticon stands up and the debris is not visible.
Continuity mistake: When Sam's parents first show up, we see a shot of their RV with a car parked behind it, on the pavement. When they go to use this car to take him to his interviews it is now on a trailer behind the RV.
Continuity mistake: When Sideswipe kicks the gun towards Ironhide, the latter one catches, turns, locks, aims and holds the gun still all in the same motion. The nest shot has him do the same again, only this time he keeps on turning.
Continuity mistake: When Optimus dives in front of Shockwave, there is all sorts of rubble around the latter one. In the shot of the bullet arriving at the cupola, all of the rubble, damage to the street and broken cars are gone.
Continuity mistake: Just before the soldiers jump out of the Osprey, there is a shot of them running for the exit. You can see a very big gun on the soldier's chest. But in every shot of them wingsuiting through Chicago, none of the soldiers can be seen carrying any kind of weapons, let alone big LMG's and assault rifles which they use after landing.
Continuity mistake: After Sam and Carly are reunited, her hand keep altering from being very dirty and in Sam's face, and clean in Sam's neck.
Continuity mistake: At the beginning, when Sam receives the medal , his hairstyle (parting) alters .
Continuity mistake: When the Decepticon jumps into the leaning building, Carly ducks in panic. When the shot cuts, she is standing straight up again, very calm at that.
Continuity mistake: When the soldiers are wingsuiting into Chicago, they fly past Trump Tower multiple times.
Continuity mistake: When Starscream flies in to destroy the Ospreys with Lennox and his team, you can see that the streets below are pretty badly damaged. When the soldiers jump, there are a few shots from a helmet cam from one of the soldiers and the streets are completely intact.
Continuity mistake: During the Mexican Standoff, Ironhide finally forces the spike into the face of one of the Dreads and lifts him up by it. Pay attention to the red car underneath: when Ironhide smashes the Decepticon's face, the windshield is fine, but when he puts him down, the windshield is suddenly damaged - before the Decepticon lands.
Continuity mistake: In the scene where Sam and Carly are trying to get into the Autobot base, Carly is wearing a black jacket, after Sam sees Bumblebee, she is wearing a brown one, and inside she has a black one on again.
Continuity mistake: When The Xantium leaves earth, it leaves a massive trail of smoke. When Sam calls Dylan, the smoke is gone only to reappear after Starscream blew the Xantium up.
Continuity mistake: When Crowbar dropped his weapons in the ''Mexican stand-off'' we get a look at his ''dread locks''. When he pulls the spike which he throws at Ironhide, the locks disappear, only to return in the subsequent shots.
Continuity mistake: In the first scene, as Carly comes up the stairs, it is bright and sunny outside. As Carly leaves, it is raining, but when Sam walks outside and sees his parents, it's bright and sunny again.
Answer: Mudflap and Skids (the two idiot robots I believe you are talking about) were removed from the film due to negative fan reaction. While we don't hear what happened to them in the context of the plot, no reason not to assume they were killed in fighting between the movies. Arcee and Chromia (2 of the motocycles) were killed in the last film.