I worked with vacuum systems in grad school and this sometimes happened if we scraped one metal part a bit too hard on another when positioning a sample inside a characterization chamber. Cue having to vent the system to extricate the sample manually, followed by at least a couple of days of effort to get the chamber back to ultra high vacuum, such as wrapping the chamber with heating pads to force adsorbed water to leave quicker.
I know it's not trivial but do organisation working with vacuum chambers ever set it up so that the atmosphere inside a pressure vessel is kept full of dehydrated oxygen/nitrogen air at positive pressure during maintenance activities?
I know they make sure that all the workers suit up to prevent oil from skin ruining any chance at a high vacuum.
Sure, it's very common to flush a system with dry nitrogen for that reason. But of course if you have to fully open a part of the system you're usually out of luck (also you usually don't have valves at every intersection etc., so some part may still be exposed to the atmosphere even if part is "kept dry").
