“Fine Doesn't Match Offense”

I was actually going 83kph in a 50kph zone. The officer was very nice and meant to knock the ticket down to 15kph...which is the fine he gave me (set fine of $37.50 and no demerit points). However, in describing the offence, he indicated I was going 75kph in a 50kph zone!!

If I just let this go, plead guilty and pay the fine as indicated, am I at risk for them coming back at me for a higher fine amount and/or demerit points?

I don't want to cause trouble for the officer but I also don't want to admit liability if it leaves me open for more problems in future.

An incorrect fine typically invalidates the ticket. If the Justice of the Peace who reviews the ticket notices the error, they should quash it right away. If they fail to notice the error, you'll have to take some additional steps, but the ticket should still be quashed. Search the forum for threads with London v. Young which talk about this.