10 Km trick did not work!!
Here are the details:
I got a 100 % Eevee, as I have enough Vaporeons, I decided for an Espeon. I already got 3 Espeon using the 10 km trick (plus one with the name trick). I walked it as my buddy for 11,4 km. I got the 2 candies as espected. I evolved it from the buddy screen and surprise surprise, A vaporeon appeared.
what could have gone wrong?
has it happend to any of you?
I double checked the distance, it says 11.4 km, as for some people there is a bug about the distance, that after evolving. the distance change to less than 10km. I was in the Subway, reason why the GPS was not accurate, but the the distance had already been updated and already got the candies.Could it be possible that going in the subway (speed and not accurate GPS signal) could have affected.

Answers
Well first, at least it's a Vaporeon. Highest CP option for your 100IV, probably better than Espeon in my opinion.
As for what happened:
You probably didn't have a signal in the subway (you had a signal when you started to evolve but not when it was deciding who to get, probably). Thus, the game didn't know whether it was day or night (or didn't know that you had walked 10km since it had no signal to check with). As a result it opted to just do the random evolve.
Just a guess, but I would imagine the fault was lack of signal.
If you were in a subway then chances are you lost GPS tracking while you were evolving. There have been reports of trainers getting the original three due to poor or disabled GPS.
Without geolocation, the game could not decide where in the world you were, and thus could not determine if it was day or night. Logical but not reasonable, just the way machines are.
No GPS = No way for the software to be sure what time it is. It cannot rely on your phone clock, because you could simply set it to any time you want, no matter what time it is (ex.: It's night, you want an Espeon, you set the time for noon). So, since you likely lost GPS, you got a Vaporeon. Next time, just make sure your GPS is getting a good signal.
Same here, I got a 98% IV 800 CP Evee and kept her as buddy for 10 km. Then I evolved at 11 pm to get an Umbreon, but a Flareon came out. My GPS signal was stable at that time.
My girlfriend also caught the same Evee and does not dare to evolve her anymore, before its clear what exactly caused this error.
Has anyone found out more about this issue in the mean time?