You'll want to be careful with how you shop online for the holidays, or retailers could end up spoiling your gifts. Through a type of ad tracking called "ad retargeting," retailers can remember what products you've bought and browsed on their websites, and then advertise them back to you across the web. At Marketing Land, Danny Sullivan writes about how ad retargeting nearly spoiled his wife's holiday purchases this year. Items that she looked at on Macy's and ThinkGeek kept turning up in banner ads — a big problem since their children shared the computer.

Unfortunately, opting out isn't always possible. Marketing Land reports that while some ads allow it, for others, the only way to hide a product is to start browsing the advertiser's website again until it promotes a different set of products. Gift buyers can get around it in the first place by doing their shopping through a private browsing mode or a personal account on a shared computer — but if they don't, family may be left wondering why their gifts look so familiar this year.