Is it possible for a bolt to come loose inside of a hydraulics system?

A bolt inside the hydraulics system of a Caterpillar blade unit destroyed the pump, and the mechanic is blaming a hydraulic oil additive that was also in the system. From what I know about hydraulics design, this simply is not possible. I think it much more likely a stray bolt got inside the system during overhaul or filling, but I do not want to dispute this report and anger the hydraulics mechanics, if it is even REMOTELY possible that Caterpillar designed their system with an internal bolt that could eventually come loose.

SO...is there ANY hydraulics system design that includes a bolt INSIDE the system? (I would think the possibility of liability would be extreme and such a design would never be approved, but I've seen worse...)

Thanks in advance for any answers!