Just for info, it can happen... and it is not very serious...
I was wearing my 345.0022 during weekend, and noticed in the afternoon that hour was completely wrong... hands showed 4h30, but it was 3h15 in real life...
Set hour again, checked... and it was obvious that the seconds were going really too fast!
Chrono was working well, everything Ok, except that time was too fast...

Today at my watchmaker: the balanced wheel spring was "blocked" by a very small pin (see pic; it is not "my" caliber).
I'm not exactly sure about the pin (only a very quick description by phone... not easy when you don't have the caliber in visual).
Origin: can be a shock... but a shock at a very specific bad time, when the spring is totally loose (middle position). I really don't remember of such a shock; I wear it very very carefully... but that happened...
So if it happens to you, don't worry...

 

Your watchmaker told you what happened. If you are questioning it, does that mean you don't trust his answer?

A shock to the watch, at the right time and in the right direction, can cause the spring to get caught up on the stud, like is shown here:



Or here:



It happens, and it doesn't necessarily take a big shock, so it's very possible that you bumped it and didn't realize it.

Cheers, Al

 

Oh, super great pictures... excuse my english, sometimes, it causes misunderstandings... I trust him, of course... but I did not talk to him directly, I was far away and let the watch to my father who took it to him. I think you have the explanation on your pics: this is probably the "pin" my father was talking about I guess, "pin" is the only word he understood from the watchmaker...
But your pics are not from a 861, exact?...