Version 1.6.5 on nginx and varnish
When doing bulk smushit on a media library of 15 pages I got:
Error 503 Service Unavailable
Varnish cache server
Each time it would process part of the library then throw the error, so I just kept rerunning bulk smushit till it finished.
I've submitted a ticket to our infrastructure support people to see if they have any feedback on the particular error but I was able to finish the process after running it 8 times so it's not a high priority.
I thought I'd share this on the forums and I'll post any info I get from our infrastructure people when I hear back.