Does it often happen?

https://scontent-fra3-1.xx.fbcdn.net/hphotos-xla1/v/t1.0-9/12512649_1385137388169371_2720957211705003252_n.jpg?oh=e26ac1101c02fb57f417090e7e8cb9d6&oe=57B4FC80

No.

My copy of Descent: Well of Darkness came with one fewer ferrox and one extra kobold. It happens...rarely...

Bonus: I've lost a life counter somewhere....

I don't use those counters. 6sided colored dice work much better