A contradiction in terms is a couple of words, usually comprising an adjective or two and a noun. Many are satirical, many light-hearted, but the best are cynical. Here's a selection for your perusal.

• Act naturally
• American English
• Artificial reality
• Baby grand piano
• Black light
• Bottoms up
• Bureaucratic efficiency
• Bus timetable
• Cardinal sin
• Charm offensive
• Childish innocence
• Civil war
• Constant change
• Cost free
• Criminal justice
• Down escalator
• Dress pants
• Dry rain
• Dry wine
• Elementary calculus
• European Space Agency
• Falling between the cracks
• Freezer burn
• Fresh frozen food
• Friendly argument
• Friendly fire
• Fun run
• Fuzzy logic
• Good grief
• Gourmet pizza
• Great Depression
• Head butt
• Holy war
• Hopelessly optimistic
• Idiot savant
• Industrial park
• Irate patient
• Jumbo shrimp
• Land of the Free
• Liquid crystal
• Live recording
• Management - they rarely manage to show up, let alone manage anything worthwhile
• Managing director
• Mature student
• Microsoft Works
• Near miss - a near miss is a hit, isn't it?
• Old news
• Original copy
• Partially complete
• Passive resistance
• Plastic silverware
• Politically correct
• Postal service
• Pretty ugly
• Random selection
• Rap music
• Second best
• Sensitive male
• Silent alarm
• Smart bomb
• Social science
• Soft rock
• Standard deviation
• Stiff upper lip
• Student teacher
• Sweet sorrow
• Tactical nuclear missile
• Terribly nice
• True lies
• Unbiased opinion
• Working vacation

A contradiction in terms is closely related to the oxymoron. They may indeed be the same thing, they may be drastically different, or they could even sometimes be so similar that only people with too much time on their hands will argue about which is which.