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I have only seen a car with sheared bolts once, and that was because I think someone was trying to do the valve cover themselves and didn't know what they were doing so they brought it in to our shop. The bolts are aluminum and should be replaced for proper torque whenever the valve cover gasket is replaced.
This has been a routine job for any e90 with around 60-70k or more miles at our local service shop, on average once a month or so.
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