If you are ever considering pantsing a friend, double-check it's them before hand.
This is one of those awkward moments that happens about once every 3 months, when you haven't gotten enough sleep recently or are just a little more spaced out than usual. You're chilling with your friend in a deli, or in the movie theater line to get your popcorn, or something of the sort. You turn to your friend and start to say some kind of inside joke like, "yo I love how whenever I come here it's exactly like-" and you immediately cut yourself off when you make eye contact and realize that it wasn't your friend but a complete stranger you said that to. Your friend had gone to pick up a soda or to run to the bathroom without telling you and somehow, without you noticing, this rando slipped into your peripheral radar right where your friend was and your lack of attention and their similar looks (perhaps they were both wearing red hats, or a puffy jacket), made it so you didn't even realize that a switch had occurred.
Usually when this happens, I am so shocked and confused and awkwardized that I just fumble with my words for a few seconds and mumble something to the extent of "sorry, thought you were my friend." (Hopefully, this doesn't happen in line at the movie theater because then you are stuck standing with that stranger alone in the awkward and silent understanding of what just happened until your friend comes back from the bathroom. Unless avoiding this kind of awkwardness means enough that you are willing to forgo popcorn to avoid seeing this man again.) It's always awkward when you say something overly personal or some kind of inside joke that only you guys understand because then the stranger is wondering why you would ever say something so strange or why the two of you would ever find it funny, because inside jokes never make sense to outsiders.
For example, my friends and I find it funny to go up to each other from behind and say stuff in a weird hillbilly voice like "hayyyy whatcha doinnnn." (don't ask why- see how not-funny it is when explained to a third party?) So my friend went up to our friend from behind in the dinning hall and did this in the sketchiest, creepiest voice imaginable, only to realize it wasn't his friend. Ow. He said he felt so awkward he just walked away and pretended like it didn't happen. Objectively awkward.
About a week ago I was in a bookstore line getting ready to pay with my friend. I saw a really interesting quote inside of the book on the front counter and read it aloud to my friend (about a sentence or two). At this point it's pretty obvious, isn't it. My friend was 25 feet away and the person I thought was my friend just happened to be wearing a similar jacket, hence my mistake. My move was unconventional but perhaps prudent: I pretended like it was all on purpose. As soon as I realized he wasn't my friend I finished the sentence and acted as if I was reading it aloud to myself. I then nodded my head in an exaggerated agreement with the wisdom the quote provided (and provided especially when read aloud). I think that guy thought I was crazy. At least he didn't think I was awkward.
The ultimate manifestation of this awkwardness happened to a friend of mine, Chris. Chris was in a public swimming pool with his best friend Dave. Chris loves pranking people and thought it would be funny to pants Dave (pull down his swimming suit while he was underwater, exposing his genitalia to the public- once again, not as funny when explained to a third party). So he swam up behind Dave (sneak attack usually works best for pantsing) and yanked his trunks off. Once again, it wasn't Dave. A 40-year old man pops out of the water and yells "what the fuck do you think you're doing?!?!" Chris stutters something like "sorry, I thought you were my friend" and swims off to avoid ever seeing this man again. Awkward to the point of pain. I wonder what that man thinks of Chris; even if he understands it was an honest mistake of identity, he still probably wonders if he's homosexual or just plain weird, given the fact that he enjoys ripping off his friend's pants in public. But after you pants someone it's pretty hard to give the explanation that you just thought they were your friend...