Had a brain fart and topped the car up with 87, so I guess I'm mid grade now.
Any worries seeing as I drive like Grandma most days and generally top it off once a week, or around half a tank of use?
If the car isn't tuned, it's a non-issue. Take it easy as a precaution and make certain to fill up on premium unleaded fuel next time. FYI - Per Honda, required for fuel the Civic Type R is Regular Unleaded; however, Premium is recommended.
I wouldn’t worry. I also did the same because the gas station I went to had octane in reverse. I drive pretty tame on the street and the computer will sort it out. Just keep the the low. I would be more scared with a tune.
it will pull timing/boost when it detects knock to protect itself. not ideal, especially in the summer heat, but shouldn't cause any damage. i'd drive with a light foot.