git push -> rejected?

I'm still not that used to git, but I'm trying to use it as much as I can, and so suddenly I crashed against this:

$ git push
To git+ssh://...
! [rejected] summary -> summary (non-fast forward)
error: failed to push some refs to 'git+ssh://...'

Mh, "not-fast forward" what? am I playing a song? well, after all, I didn't do anything strange (I suppose):

• was on master
• branched out a feature branch
• did some commits on master and some commits on the branch
• rebase the branch on master

all (except rebase) from a couple of machines on the same repo.

Well, after a bit of panic, I googled and found not to worry: did another pull

$ git pull
Merge made by recursive.

then push went pretty good:

$ git push
...
To git+ssh://...
6f6324d..f39911e HEAD -> summary