A wave of giant IPOs led by SpaceX is heading for retirement accounts, and it could be bad news for anyone whose 401k is quietly buying in at the top.

• SpaceX is set to go public at a valuation that would make it worth more than every American defense contractor combined, the biggest IPO ever.
• Rules meant to keep retirement money out of overpriced, hard-to-sell investments were loosened right before these deals.
• That means 401k and retirement funds may end up buying the shares that early private investors want to sell.
• Ordinary savers get the risk without ever choosing it, since the buying happens inside funds they already own.
• Prices this high on day one leave little room for gains and plenty of room for a fall.

Outlook: Expect the IPO rush within weeks, with retirement money as a major buyer and growing worry that this is a bubble forming.