Bug#838561: (no subject)
I have got the same problems.
I am getting a lot of random "503 Service Unavailable" while using snapshot.debian.org.
For me snapshot.debian.org resolves to 193.62.202.30 and 185.17.185.185
I also tried do isolate the problem to one of these IPs by adding a line to /etc/hosts
but it turned out that both server respond with "503 Service Unavailable" very often.
But if if you repeat the download several times it will sometimes succeed,
at least with curl.
To reproduce run:
watch curl -I "http://snapshot.debian.org/archive/debian/20170413T031942Z/pool/main/a/apt/libapt-pkg4.12_0.9.7.9+deb7u7_amd64.deb";
This results in a lot of "Size mismatch" during "apt-get dist-upgrade" e.g.:
Failed to fetch http://snapshot.debian.org/archive/debian/20170413T031942Z/pool/main/a/apt/libapt-inst1.5_0.9.7.9+deb7u7_amd64.deb Size mismatch
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