[Tristan Lall]

Not so fast. It's also a spare part, which doesn't count against the limit.

That's a symptom of the real problem--a badly conceived rule. There's no universal way of determining whether their extra drill is a spare part, mounted on the mechanism for convenience (where it is legal) or a part of the mechanism (where it is illegal, due to weight). You could take the naïve approach, and say, "well, it's attached to the mechanism, so it mustn't be legal", but that comes down to a silly semantic argument as to when something is part of something else, and when it's a separate entity. Note that they (properly) pass the weigh-in, because they know that a mechanism or part need not be attached to the robot to be part of its official weight (e.g. the two function modules--which probably can't be attached at the same time).