(Slightly related) Losses as percentages always annoy me - a 2% loss is not made up by a subsequent 2% gain.
What bothers me are per share prices. I don't remember the precise value of any particular stock, so IBM drops $13 means nothing to me. Also that a stock is $500 a share, so it must be good is another thing you see a bunch, so it must be great is another fallacy I see all the time; when really all that matters is the market cap not how many pieces it is divided into.
Edit I think it was $200. See numbers matter.