I heard that the fatal mistake french planners made during the german invasion of the low countries in ww2 was that the Ardennes was essentially impassable. How were the Germans able to cruise through the forest and surprise the French with their pants down?

There's much more that can be said. Mentioning previous discussion is intended as a starting point, not to discourage more questions, info, or debate.

There's an FAQ answer to "How did german tank armies manage to traverse through the ardennes forest during Fall Gelb? Something that was deemed impossible by the allies." at https://www.reddit.com/r/AskHistorians/comments/2cvgu8/how_did_german_tank_armies_manage_to_traverse/cjjq771/?context=3 by /u/kieslowskifan . You can click on the title to see the other responses.

There's also "Why didnt the French know that you indeed could drive tanks through the Ardennes forest?" at https://www.reddit.com/r/AskHistorians/comments/786mr6/why_didnt_the_french_know_that_you_indeed_could/ . kieslowskifan polished a related previous answer, and /u/Avatar_exADV , /u/Toxicseagull , and The_Chieftain_WG had some details too.