If your share count hasn't changed, then all they did was maybe reduce your future capital gains taxes by increasing the total cost of some of your shares! lol As long as you know what you paid and your total share count, at least you're not being ripped off, unless they jacked up some of your commission. If you did some pre or extended hrs. traded, you could've paid higher commissions on some of your trades which would increase total cost. I don't believe the IRS can come down on you for your brokers faulty data, but getting surprised with upward revised tax liability is no fun either, so I agree you should contact them and try to get it resolved.