josepp, Yes the ducts they are always wrong.

In a large building they can be up to six feet square at first, but when they get to the individual rooms they are very small. I've built some that were 4 inches by 3 inches.

Hardly big enought to crawl through.

Then on the inside they are joined together by sharp sheet metal screws that are screwed in from the outside leaving their sharp points pointing inwards. So if you ever got one to crawl through you'd get torn to pieces at every joint.

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Then there's the welding or burning (oxy-acetylene cutting) it is always wrong. In The Full Monty they showed someone arc welding and commented that the gas mixture was all wrong. There is just the possibility that they were trying to make the character seem ignorant but I don't really believe that.