Different repository root than its parent

Fairly new to working with branches in svn, I'd been merging files from trunk to branch like this for some time:

Imagine my confusion when all of a sudden, for no obvious reason, a merge fails with this error:

After some googling, I came to the conclusion that there must be some discrepancy between the url I was using to access the repository and the url I had used to check out my working copy. I didn't understand how this could possibly be since previous merges had worked, but checked anyway:

There it is, same hostname -- all the articles I found on google were ppl having this problem because they mixed-and-matched different hostnames which resolved to the same machine, confusing svn. Just a sanity check:

As a last resort, I tried explicitly specifying my username, the way it was in the output of svn info. This worked.

It then occured to me that this was the first time I'd ever tried to merge the addition of a directory. Still somewhat baffling, but apparently svn is more picky when merging changes to directories than when merging changes to files. Very strange. Hope someone else will waste less time on this than I did. :P

Oh, and so I never have to go through this again: