Maybe mishandling error cases?

[peterbourgon]

https://github.com/tsenart/vegeta/blob/master/lib/attack.go#L62

if body, err := ioutil.ReadAll(r.Body); err != nil && result.code < 200 || result.code >= 300 {
    result.err = errors.New(string(body))
}

This will set result.err if err == nil and result.code == e.g. 301 (I think a false positive) and will not set result.err if err != nil and result.code < 300 (I think a false negative).

Maybe I misinterpret your goals here though.

[tsenart]

Nice catch. Thanks.