This has apparently been happening enough that it's part of the code for the test:

https://github.com/ssbc/go-ssb/blob/master/message/legacy/verify_invalid_test.go#L20

Where it's crashing is here:

https://github.com/ssbc/go-ssb/blob/master/message/legacy/verify_invalid_test.go#L82

So...I think we've got some data in testdata-fuzzed.json which is invalid.

See #237