The antique glasses that Admiral Kirk sells to make some cash are the pair that was given to him by Dr. McCoy for his birthday in Star Trek II - L'ira di Khan (1982). It's suggested that once sold in the antiques store, those glasses hang around until they are bought by McCoy, in the future, and then Kirk takes them back in time, and so on, in which case one has to wonder where the glasses "originally" came from. This constitutes an "ontological paradox", an old favorite of science fiction writers, and raises too many questions to discuss here. (It's possible that these glasses existed in two places simultaneously, like characters in Ritorno al futuro (1985) and its sequels, rather than being caught in a causal loop.) The same paradox arises when Scotty helps Dr. Nichols "invent" transparent aluminum. If the formula is "found" for the first time in the twentieth century, but only because Scotty took the information back, then this was never invented in the first place! (This may not be a paradox once Scotty only gave Nichols the chemical formula but not the manufacturing process.)

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