This issue comes up with a reused bytes.Buffer.

    var buf bytes.Buffer
    buf.WriteString("foo")
    b := buf.Bytes()
    fmt.Printf("b == %v\n", string(b)) // b == foo
    buf.Reset()
    buf.WriteString("bar")
    fmt.Printf("b == %v\n", string(b)) // b == bar

Yes the documentation for .Bytes() says that "The slice is valid for use only until the next buffer modification" but people don't always read the docs for every method they use, especially if it's a method they've used a bunch before. Having a reusable buffer that you return the .Bytes() value from is very tempting. Bug-free code would either copy the result or not reuse the buffer.