Lambda sensor bad?

[ronenfe]

Took me a while to find the nut, I was worrying it can get inside the engine, but luckily there are no big holes there, and it didn't break anything above, it was in the hole of the same push rod, I realized it after I saw the push rod was not moving freely. I had to take out the whole valve shaft and the push rod and it fell inside the hole, I managed to dig it at the end with a magnetic screwdriver but it wasn't easy.
Maybe it happened because the nut wasn't tighten strong enough.