There's your problem. Your body was ejecting all the ketones in your pee instead of burning them for fuel after you had that McFlurry. Pee ketones are notoriously inaccurate. You shouldn't be peeing out your fuel source once you're fat-adapted.
No I don't think it's a bad thing, just that having ketones in pee doesn't necessarily mean you're in ketosis (especially after a sugary snack). Lots of people stop seeing the peetones after they're fat-adapted, but doesn't seem like something to worry about if you still get them.
But if you stop seeing them, and then see them again after a binge, it means you got kicked out of ketosis.
I'm sure most of you are aware of the PTA Banting market, but for anyone starting out, the first one for 2019 is tomorrow at the PTA Botanical Gardens. I have found quite a few things at the market that have become staples in my pantry. Also Bacon "donuts"