I shall post an experience I've had not too long ago, very recent, in fact since the calender date just changed.
After discussing with another fellow CS-er, decided to post this so all who go for outings can be more mindful of such incidents.
Given the benefit of doubt that it might be an honest mistake, I shall not disclose the other party's nick and name in this post. I shall refer to him as MrQuickHands. Don't think there is a member here by that nick, if there happen to be, it is purely coincidental.
The following is a step by step illustration of the event, in plain straightforward descriptions.
1.) MrQuickHands and I had the same lens (Canon 100mm). Mine is almost new at 1 month old, his is of an earlier generation.
2.) His lens is not mounted on the camera, it is on the table. Mine is mounted on my camera.
3.) MrQuickHands took about another lens, a Tamron and asked me to try it.
4.) I'd dismounted my Canon lens, mounted his Tamron.
5.) Since my lens is on the table too, just put his Tamron rear end cap onto my Canon lens.
6.) At this moment, there are 2 Canon 100mm lenses on the table. His with Canon end cap and mine with his Tamron end cap.
7.) As I tried shooting some shots through the camera, I saw MrQuickHands swap the end caps on the 2 Canon lens.
8.) Next, MrQuickHands put the one with the Canon end cap (mine) into his bag. There is only 1 Canon lens on the table. Magical.
9.) I dismounted the Tamron lens and returned to him.
10.) I mounted the one and only Canon lens onto my camera to see if he will voice anything. He was very helpful. He reminded me to check that the red dot on the lens must be aligned properly with the camera mount. Nice guy.
11.) After I mounted the lens, nothing came from MrQuickHands. Peaceful. So I made it really simple by looking at him and gently said "Oi! Wrong lens!".
12.) MrQuickHands then apologized for the mix up and retrieved my lens from his bag and returned to me.
Maybe it's a simple misunderstanding, maybe it's a performance of sleight-of-hands gone wrong, I don't know.
Let's assume MrQuickHands really didn't realize he swapped the end caps on the lenses, and then subconsciously put my new lens into his bag. No biggie, I got my lens back in the end.
Moral of the story, be mindful of your equipments and those around you. It might be a silly mistake, but don't let such incidents happen and become an unknowing and accidental victim.
Thank you for reading.