First fo all, Spock would never say, "I thought you said that repair would only take you an hour." In fact, it's as unSpocklike as can be, as is, "Well how long will this take?"
Also, I never saw Scotty give an estimate he had to back of like this. Assuming it did happen, in-character dialog would go more like this:
SPOCK: Mr. Scott, you stated that the repair would only take one hour.
SCOTTY: Aye, sir, that's when I had assumed theó
SPOCK: Assumptions are not not based on fact, Mr. Scott. I require a realistic figure.
SCOTT: Twelve to fifteen hours, sir.
SPOCK: That is considerable margin of error. Explain.
At which point Scott better damned well have a "logical" explanation.
^The obvious next line by Mr. Scott would be, "Aye, sometimes there's a lot of error involved, sir."
These, work just nicely and cut down on the babble and give a good badinage we're used to from TOS.
If you really must, maybe the Antimatter induction valves, it always seems to be something wrong with the antimatter in TOS.