Patch within branch

Hi all,

I appended this question within a reply to an earlier post but then decided
it deserved it's own thread.

I have a patch (actually 2 different patches) that I use to maintain the
differences between 3 separate installations of the same product. I came
across an issue recently wherein I discovered that if I update the patch
file in a branch and then merge everything back to the trunk, and make a
change to my application in the trunk, I'm unable to apply the patch. I
receive an error indicating that the patch seems out dated. I was able to
merge my changes back to the branch and then apply the patch. There was a
conflict which I needed to resolve but this was expected.

I don't want to lose the ability to apply a patch after the changes have
been merged back to the trunk and the branch is blown away.

Can someone help me better understand why the patch is dependent on the
branch? Is this a bug?

-- 
Kerry Carroll
carrolky_at_alumni.uc.edu