Hey,
when the list of cars in your ‘Car Stock (Showroom)’ can not be expanded anymore so the last car will be shown in the buttom slot of the list the tool tip for the car will be shown while moving the slider an may hide the Price and Premium/Discount:
Others mentioned that the slide may exceed to the right in case of lowering the basic price after finish of upgrade research and use in production. It would be better to keep the slider at the position of 100% even if the Premium/Discount is higher than 100%:
Kind regards
[Edit] The 2nd point was already posted: ‘[Open][Bug 1.10] Cars cant sell (Pictures) / Buggy Stock (Showroom) price slider’
http://positech.co.uk/forums/phpBB3/viewtopic.php?f=57&t=18306