Mobileye didn’t need the help of Elon Musk to rev up its stock. But investors are wise to pump the brakes now.

Mobileye’s shares skidded on Friday. Mostly, this was part of a sell-on-the-news reaction to Tesla Motors unveiling a new “autopilot” system for its current line of Model S cars as well as a new “D” trim that adds dual electric motors for improved handling.

Mr. Musk, Tesla’s social-media savvy chief, sent out a mysterious tweet on Oct. 1 to tease the announcement. This gave Mobileye’s stock a short-term boost to a new record high. But the shares already had more than doubled from their $25 initial-public-offering price in the two months prior.

Tesla confirmed Friday that Mobileye is a supplier for its new system.

Mobileye’s EyeQ chips use cameras to scan the areas around a vehicle and alert for potential collisions. The company expects about 160 car models will include its chips by the end of this year.