have seen this failing on Travis with:

**** v1   vsl|       1004 VCL_call        b BACKEND_FETCH
**** v1   vsl|       1004 VCL_return      b abandon
**** v1   vsl|       1004 BereqAcct       b 0 0 0 0 0 0

Tested ok locally with varnishtest -n 1000 -j 60.