You can't win, can you? I had taken my oscilloscope apart to see what I could do about the somewhat noisy fan. I knew I lost one small nut, but couldn't find it anywhere. It must have fallen on the floor. To make sure it wasn't stuck somewhere in the device, I turned it both ways, knocked it, shook it and manipulated the power switch a couple of times. I had a box of the same sized nuts around, so replaced it upon reassembly. I fastened the inner shielding, turning the device on all side several times, fighting a few screws that wouldn't go back in and fastened the outer casing. Finally, after all was said and done, I put the oscilloscope back on the desk, turned it on and immediately heard a nice clink of the missing nut inside the inner shielding of the device.
Long story short, I took my oscilloscope apart twice today.