Strange result of forced merge

[Michal Krause]

Hello,

we use Mercurial for development of midsize projects in team of 6 
developers. Two times one developer used hg merge -f (because he had 
outstanding uncommitted changes) and result of this operation was that 
all changes made by other developers between both heads were completely 
lost in repository, but they all appear in developer's working copy as 
uncommitted.

I know that merge -f is potentially dangerous operation and it probably 
should not be used, but I'd like to know what can be the real cause of 
such result of merging, because developer who made it swears that he 
didn't made anything special and all my attempts to simulate it in 
testing environment failed (all merges including forced worked as expected).

By the way, is there a chance that Mercurial will allow merging with 
uncommitted changes if they don't conflict with both heads being merged?

Best regards
-- 
Michal Krause
Internet Info, s.r.o.