Is it possible to power an AC-to-DC switching adapter with a DC input?
Powering such a device from DC is perfectly fine, in fact usually the input is rated both for AC and DC. You just need to be very careful about the input voltage though: when you rate an AC input you usually give the maximum peak to peak voltage amplitude, but when you rectify it there is that 1.414 factor, also known as \$\sqrt{2}\$, that must be taken in account.

So, no risks for the user or the adapter. If you exceed the maximum input voltages some capacitors are going to explode (at least), if you use a lower input voltage the adapter won't work properly and might heat up due to undervoltage protections.

I'm guessing you want to power your laptop with your car battery: that is possible but not just hooking the laptop adapter to the battery.