If you want to be able to creep people who know on Instagram without running into the dreaded accidental like if a picture posted an embarrassing amount of time ago, InstaSnoop is the app you need, pronto. Especially if it's a habit of yours to lurk old photos, the free iOS app needs to be on your phone.
InstaSnoop allows you to stalk people on Instagram -- from your ex to a guilty pleasure celebrity -- without having to worry about hitting the like button or leaving a comment without meaning to. This will save you a lot of embarrassment and keep the person you are looking at unaware of your activities.
You can also add people to your Snooplist to check profiles quickly. There is an added zoom feature as well -- something that doesn't exist on regular Instagram.