forced update of master branch?

[Marcus Shawcroft]

Twice this morning I've pushed changes upstream which have
subsequently been removed when someone has done what appears to be a
"git push -f" to an older version of master.

Of course I may been doing something stupid, but if I'm not can anyone
shed any light on why patches are being dropped  from master without
email traffic on the topic?

Cheers
/Marcus