If you have Magento 184.108.40.206 (this may apply to other versions too), apply the SUPEE-10415 security patch, and then use Git to merge the result into a branch with Magento 220.127.116.11 in it, then your site will break if you use Git's default settings.
This is because the security patch and Magento 18.104.22.168 declare
in different places in the same file, and when you merge with Git's default behavior, it keeps both declarations, which in principle is how you want it to behave - after all, git can't be expected to know very much about PHP syntax, and for all Git cares it's just two code hunks that got added to the same file.
I would add an answer to Security Patch SUPEE-10415 - Possible Issues? but I don't have enough reputation to do so. So I'm choosing to add a question and link to the original instead. I'll also add an answer on how best to fix the problem for those of us who find out about this issue the hard way.