The climax of the final battle. Achilles blows up Alexander's robot by firing his own Rocket Punch into it! It stretches the Willing Suspension of Disbelief since Achilles took the same blow to the chest in their first fight and his robot was just thrown off-balance, but Rule of Cool demands it be allowed anyway, consistency be damned.
The book -derived from an expanded version of the film script- establishes that the rocket fist was supposed to be explosive, but failed to detonate when used on Achilles due to an unspecified defect that would have been fixed by the second battle. The fired hand sitting next to Alexander's robot as it explodes is a filming error.