Shares of aerospace start-up Lilium ( LILM 0.82% ) fell as much as 13% for the week, according to data provided by S&P Global Market Intelligence, after a rival warned that regulatory approval for its so-called "flying car" aircraft would take longer than it had hoped. This is a brand-new industry, and no company is likely to get a free pass, causing concerns about Lilium's somewhat fragile balance sheet.