Slow-Slowly

I have walked by this vending machine countless times. This machine sits in a row of other vending machines. Some of the machines have written on them, "Push in and pull out SLOWLY." The red sign has a mistake because the word SLOW should be followed by -LY. When a describing (modifying) word tells how and tells about a verb, it is an adverb. Adverbs often end with -LY.
Examples:
1. The slow car is red. (Tells what kind of car and doesn't need -LY.)
2. The red car moves slowly. (Tells how the car moves and needs -LY.)