Varnish intermittent 503 in Magento 2

Question

I've followed the dev docs on setting up varnish, and increased the http_resp_size and http_resp_hdr_len when the daemon starts. Most of the time it's fine, but every once in a while I will get a backend fetch failed. (probably one every few thousand requests, very hard to replicate)

Checked using varnishlog -d -q "BerespStatus == 503"

There doesn't seem to be much indication of an actual problem other than the error I'm seeing:

FetchError HTC status -1

One such failure is as follows:

<< BeReq    >> 426839
Begin          bereq 426838 pass
Timestamp      Start: 1534283980.206907 0.000000 0.000000
BereqMethod    GET
BereqURL       /
BereqProtocol  HTTP/1.1
BereqHeader    Host: <omitted>
BereqHeader    Pragma: no-cache
BereqHeader    Cache-Control: no-cache
BereqHeader    Upgrade-Insecure-Requests: 1
BereqHeader    User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/68.0.3440.106 Safari/537.36
BereqHeader    Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8
BereqHeader    Accept-Encoding: gzip, deflate
BereqHeader    Accept-Language: en-US,en;q=0.9
BereqHeader    Cookie: mage-translation-storage=%7B%7D; mage-translation-file-version=%7B%7D; form_key=cm37N1pWTHh0CP1o; mage-cache-storage=%7B%7D; mage-cache-storage-section-invalidation=%7B%7D; mage-cache-sessid=true; mage-messages=; recently_viewed_product=%7B%7D; r
BereqHeader    X-Forwarded-For: <omitted>
BereqHeader    X-Varnish: 426839
VCL_call       BACKEND_FETCH
VCL_return     fetch
BackendOpen    25 boot.default 127.0.0.1 8080 127.0.0.1 55734
BackendStart   127.0.0.1 8080
Timestamp      Bereq: 1534283980.207162 0.000255 0.000255
FetchError     HTC status -1
BackendClose   25 boot.default
Timestamp      Beresp: 1534283980.224352 0.017445 0.017190
Timestamp      Error: 1534283980.224370 0.017463 0.000018
BerespProtocol HTTP/1.1
BerespStatus   503
BerespReason   Service Unavailable
BerespReason   Backend fetch failed
BerespHeader   Date: Tue, 14 Aug 2018 21:59:40 GMT
BerespHeader   Server: Varnish
VCL_call       BACKEND_ERROR
BerespHeader   Content-Type: text/html; charset=utf-8
BerespHeader   Retry-After: 5
VCL_return     deliver
Storage        malloc Transient
ObjProtocol    HTTP/1.1
ObjStatus      503
ObjReason      Backend fetch failed
ObjHeader      Date: Tue, 14 Aug 2018 21:59:40 GMT
ObjHeader      Server: Varnish
ObjHeader      Content-Type: text/html; charset=utf-8
ObjHeader      Retry-After: 5
Length         283
BereqAcct      881 0 881 0 0 0
End