The motorway vignette is not optional
Slovak motorways require an electronic vignette, bought online or at border filling stations before you drive on them. There is no toll booth to remind you and enforcement is by camera; fines run into the hundreds of euros and are pursued across borders.
Hire cars sometimes come with one and sometimes do not. Check at the desk rather than assuming, because the liability follows the driver.
Tatras mountain rescue is billed
Mountain rescue in Slovakia is run by Horská záchranná služba and the cost of a callout is charged to the person rescued — a helicopter evacuation runs to thousands of euros. Insurance covering mountain rescue is effectively mandatory rather than advisable.
The High Tatras are a genuine alpine range despite their small area. Several high routes are closed by law from November to mid-June, and the weather turns faster than the modest altitude suggests.