Accidentally put Unleaded 88 instead of Unleaded 87 into my car

It's perfectly fine. It's actually better because it's higher octane, just more expensive.

Totally fine - Any vehicle that recommends the use of 87 octane can use a higher octane if the owner so chooses, there just may not be any benefit from it because the car's is optimized/ tuned for regular unleaded. Placing 88-93 won't do any harm....if anything it could POTENTIALLY prevent some pre-detonation. Stoichiometry for 87-93 octane is all the same so your car will run fine. That said, if your car is tuned to run on 91 or 93 octane fuel, running 87 octane is not a good idea. Most modern car's have advanced knock detection abilities and will be able to to detect pre-detonation and retard timing to prevent any damage, but it's not a wise choice.