Because my local mirros has local download speed about 700 kB/s then I
listed it on first position in my sources.list. But it isn't realiable -
sometimes it has incomplete (broken) or missing files. In my sources.list I
have other mirrors.

When apt-get didn't found a file in XXX server (even if it is listed in XXX
packages file) than it downloads from other, OK.

In other case - file is on XXX server but it is corrupted - apt-get
doesn't try download it from other mirror.

Mirek