Continuity mistake: When Tony and Tia are being driven to their hotel, the same footage is shown behind them a few times e.g. A truck turning off the road when their driver shouts at an overtaking driver and a few minutes later when he shouts at the same driver again.
Factual error: It was mentioned the museum has $3 million in gold bars. At 1978 prices, that is about 280 pounds. Gold is fairly heavy and the size of the bar shown in the film looks like 25 pounds. Assuming about 25 pounds per bar, I counted 134 bars or 3,350 pounds or almost $36 million dollars. I guess a big stack of gold bar looks better than a small stack of 11 bars. (00:41:55)
Revealing mistake: At the beginning, as the flying saucer lands at the stadium, the shadow the ship cast doesn't change size as it gets lower to the ground.
Revealing mistake: When Tony is stacking the wine barrels in Gannon's lab, you can see the wires pulling the last barrel up into the pyramid.
Answer: I always figured it was because Tony had been using his powers more often than Tia whilst he was under Gannon and his team's control, so wasn't as strong as Tia was by the time they were all together at the plutonium plant, which is why she was able to overpower him telepathically. Remember, Uncle Bene talked to Tony at the start of the film about controlling his telepathy, and that he must "never energise unless it's absolutely necessary", but the moment Tony was taken by Gannon, he was no longer in control of his telepathy, so would not have been able to ration his powers like Tia did.