Apparently, the act of liking someone's really old Instagram photo is called "deep-liking." After going "deep" into, let's say your ex's old feed, it's possible that your finger may accidentally slip and double-tap on a photo that you'd rather not let them know you're looking at.
Been there? Done that? Mhm. It happens. It's really not that big a deal, either. There's a very simple way to prevent your ex — or whomever you're lurking on — from knowing that you just spent the last two hours totally salivating over every detail of their life.
So what do you do? First thing's first, immediately unlike the photo! Of course, duh, but don't wait on it. The sooner you unlike the photo, the better chance you have of not getting caught.
Now scroll all the way to the top of their feed and like the most recent photo that they've posted. At least now, when that person gets a notification that you've liked one of their posts, it won't link back to the one from 50 weeks ago, but rather to their most recent one.
This is going to save you a lot of paranoia and it's also going to prevent you from looking like a total creep. Even if the person's most recent photo is from two weeks ago, there's a good chance that that means they don't regularly check their IG activity or have their notifications turned on, which means you'll still be in the clear.
Of course, if it's your ex, their new girlfriend, an old boss, Mike Pence's bunny rabbit, or whomever you'd rather not interact with ... you'll just have to accept the fact that that person is going to know that you liked one of their photos.
With this Instagram trick though, the person you're stalking doesn't get to know that you liked a photo from weeks, months, or years ago. Once you unlike the old photo and like the new one, your deep-like will disappear from their activity, allowing the whole thing to look more natural and save you a lot of face.