This Side of Paradise - S1-E25
Corrected entry: Spock is wearing his normal blue uniform when shot with the "happy spores" from the strange plant. The next scene is with Kirk inquiring about Spock's location to which he is told nobody has seen him. Spock somehow manages to find some green overalls out in the meadows when we switch back to him. (00:17:40)
This Side of Paradise - S1-E25
Corrected entry: A captain from the 2200's owns Samsonite luggage from the 20th century.
Correction: The original Star Trek series was actually produced on a very limited budget of about $250,000 per episode, and many of the props were re-purposed items from thrift stores. It was not uncommon to see all sorts of 20th Century items (clothing, luggage, dining room sets, household spray bottles, salt shakers, et cetera) on any given Star Trek set. Keep in mind that those items were chosen because they were rather futuristic in appearance to audiences of the mid-to-late 1960s, even though they appear whimsically anachronistic to our eyes, just 50 years later.
Also the characters have access to replicators and super computer databases. The cases could be 23rd century reproductions, classic antiques, etc.
Correction: Remember, Leila took Spock to that location for the express reason of having him infected. She likely brought civilian clothes for him, knowing he would no longer want to wear his uniform.