That happens when people are on variable rate or TOU plans, it's very common. "sneaky" may or not be part of it, since ostensibly there's a contract that defines the terms of the electrical service, so it shouldn't be a surprise. But for a lot of folks it's a lot to keep track of, there can be confusing terminology, and yes, some energy retailers are predatory in their plan marketing or contract terms. It's a double edged sword of free market choice in deregulated markets. People that have choices for their energy supply don't always have the time and knowledge to optimize their plan choices and electricity use to get "optimum" pricing. This is why there's pushback in some areas that have had deregulated energy markets to go back to regulated pricing, the "average consumer" isn't seeing the payoff of the free market (even if that is technically "their fault").

I kind of doubt a single surprise bill that happened to arrive in the winter is a TOU plan change.

If someone changes to a TOU plan and their bill shoots up, they’re smart enough to blame the plan change and cite that

Most surprise winter time bills are just excess electric heater usage, such as after the purchase of a couple space heaters without thinking about the overall cost.

> This is why there's pushback in some areas that have had deregulated energy markets

What areas have deregulated residential electricity?

The “optimum” pricing is one that rips off the customer the most. A deregulated free market for utilities doesn’t work because bad actors will find ways to do so through complex contracts.