Interesting thread.
I know of an aircraft which has a CofA but has had a couple of unapproved repairs - and for good reason. Details of how to make the "approved" repair never arrived from the manufacturer, despite asking repeatedly, and they eventually went into liquidation.
It was only a 1hr job to a seat, nothing to do with powerplant, structure, or flying controls.
I was told some "approved" repairs no longer need to be done by an approved engineer under EASA and this was one such case.
Comments welcome.