Other mistake: When Spike is thrown out of the base by Megatron at the end of the episode, Optimus calls him 'Splik'.
Other mistake: When the Decepticons shoot the monument at the end, Optimus runs to save it, you can see two Optimuses (Most likely because the animators forgot to get rid of the other prime which is just standing there).
The Return of Optimus Prime (1) - S3-E29
Other mistake: When the Protectobots combine into Defensor, his right arm is missing.
Answer: This was actually one of many errors in Transformers, the Constructicons MUST have been built on Cybertron, the only transformers genuinely built on earth were the dinobots, reason being Vector Sigma has to give personalities to each and every robot, else they end up like the dinobots, very simple and dumb. For reference watch the episode "The Key to Vector Sigma" which introduces the Stunticons and Arielbots - both Megatron and Optimus Prime have to go to Vector Sigma to get decent personality traits installed, prior to this the new robots are controlled by remote controllers and have no AI.