How did Brother Matthew almost lose his head?

From the text:

For a long time only these homely sounds were heard in the garden close, for the monks did not talk at their work.

Then it happened. The sharp chisel slipped and cut a gash across the longer piece of the cross. It broke.

Away flew the other piece as far as Robin could throw it, and after it went the chisel, narrowly missing Brother Matthew’ s head.