Something that probably shouldn't have been there (spanner, socket, whatever) has been picked up by the cam and pushed through the cover.
You'll need to remove the cover and look for something missing — if everything's in place and there's no other obvious damage then you might get away with just sealing the hole.
I've had it happen a couple of times with gearboxes — more years ago than I care to remember, the speedo drive shaft on a 1923 Dodge 4 worked loose and went through the bottom of the box when reverse was engaged.
Some years later a Lancia Lambda 4-speed box was destroyed by a woodruff key from the clutch assembly that some bright mechanic (not me — I'd never needed to touch it) had dropped inside the box and left there.
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