Another error the author made is adding to a sync.WaitGroup in a different goroutine than the one that waits. This is another rookie mistake that go test -race would probably catch.
$ go test -race -bench=.
...
WARNING: DATA RACE
Write by goroutine 4:
sync.raceWrite()
/usr/lib/go/src/pkg/sync/race.go:41 +0x35
sync.(*WaitGroup).Wait()
/usr/lib/go/src/pkg/sync/waitgroup.go:120 +0x16d
_/home/twirrim/monte.GetPiMulti()
/home/twirrim/monte/monte.go:56 +0x23a
_/home/twirrim/monte.BenchmarkGetPiMulti()
/home/twirrim/monte/monte_test.go:17 +0x62
testing.(*B).runN()
/usr/lib/go/src/pkg/testing/benchmark.go:119 +0xc0
testing.(*B).launch()
/usr/lib/go/src/pkg/testing/benchmark.go:207 +0x1ba
...