^ My mum once used oven cleaner that was so strong, the screws from the inside started coming out and shooting across the room. Maybe the oven you cleaned was more the catering type, though, so I'd assume that sort of thing wouldn't happen.
I used to clean the showerhead by leaving it in cheap vinegar. It works REALLY well, comes out like new. Except once I left it for too long and it affected the rubber.