What counts as transient errors in apt-get update?

https://bugs.launchpad.net/ubuntu/+source/apt/+bug/1693900 says

> We introduce a new flag, --error-on=any that makes apt update also error out on transient errors.

Without using this flag, _transient errors_ with `apt-get update` should return a 0 as error code.

What kind of errors are transient according to apt-get? On Ubuntu 18.04, the following error returned an error code 100.

```
E: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/bionic-updates/main/binary-amd64/Packages.gz File has unexpected size (2770332 != 2770327). Mirror sync in progress? [IP: 91.189.88.142 80]
   Hashes of expected file:
    - Filesize:2770327 [weak]
    - SHA256:93e61cef2cdc72ca9ead54c52dcfe31fe56646bc72429dcb33e9c4a163318cec
    - SHA1:44ff7061550fa1d606b8596f1a515e378d7df43b [weak]
    - MD5Sum:9b203959f5d997068ff6791bd8650773 [weak]
   Release file created at: Thu, 23 Sep 2021 16:47:21 +0000
E: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/bionic-updates/universe/binary-amd64/Packages.gz
E: Some index files failed to download. They have been ignored, or old ones used instead.
```

---

My use case is I have an internal apt-mirror. When the internal mirror or upstream goes down, I should be able to fall back to the other one (upstream or internal) automatically and `apt-get update` should return error code 0.