If you have Magento 1.9.2.4 (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 1.9.3.7 in it, then your site will break if you use Git's default settings.
This is because the security patch and Magento 1.9.3.7 declare
Mage_Core_Helper_String::unserialize()
and Mage_Customer_Model_Customer::MAXIMUM_PASSWORD_LENGTH
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.