I mistakenly put about 15 litres of 83 octane instead of 91. After realizing my mistake, I then filled in the another 15 litres in the tank this time with 91 (since 94 was not available) to mitigate the mistake. I had about a quarter tank with 91 left when I re-fueled it.
Will this cause any problems? By the way, I drive a 2010 C 250 in Canada.

 

you should have no problems at all. dont even worry about it.