I drive a lot of different vehicles when I need to get around, but I'm always a little worried when it's time to fill them up. Will something happen if I use 91 instead of 95, or vice versa? Well, this thread at StackExchange answers the question, and long story short, there's little to worry about. Photo by Mike Mozart.
Of course, the thread is concerned with a situation where, for example, you accidentally put high-octane in your tank. Ultimately, it's nothing serious to worry about (and not nearly as serious as putting unleaded into diesel vehicles or vice versa, which may necessitate you empty the tank):
Higher octane fuel does not burn as easily as a lower octane fuel. Higher octane fuels are specified where higher compression ratios are present in an engine, or where forced induction (such as turbo charging) is used. By using higher octane fuel where lower is specified, you will create no problems with your engine. It does not (by popular belief) add any power to your engine, but will not harm it at all. Worst case scenario is you've just wasted some money by buying the more expensive fuel.
If you put the lower octane fuel in an engine which specifies high octane fuel, will not cause you any major issues on an electronic fuel injected engine because it has a device known as a "knock sensor" which will pull timing.
...When you mix different octanes of fuel, you are either increasing or decreasing the octane of the fuel at hand. It won't cause any real problems for the engine or fuel system at hand (this assumes you are using fuels of the same mixture of ethanol -- mixing E85 fuel into standard fuel to increase octane and introducing it into a fuel system which cannot handle it - read this -- may cause issues with seals and corrosion of parts which are not built to take the higher concentration of ethanol. E10 fuel poses no issues for modern or older vehicles).
The full answer goes into much deeper detail on the knock sensor and how it works, and discusses what would happen if you did introduce diesel into a tank with unleaded in it already.
Either way, if you've ever wondered like I have (and flipped through the owner's manual in a car you were renting to make sure you got it right -- which you should do anyway, to be honest, if you're driving an unfamiliar vehicle), you can rest knowing you won't hurt the car -- but it's always a good idea to get it right for efficiency's sake.
What happens if you put the wrong kind of gas in your car? [StackExchange]