Mocha gives weird unformatted results where there should be test data

Oddly formatted text is printed out when I run my mocha tests. The last character is being omitted ("n" missing from "function") and I'm seeing %s: %dms after each test and several times in between each test. I cannot understand what could be happening.

$ npm run test

Result:

template.myFn() should return a string for a first element functio  ․ %s: %dms

․ %s: %dms ․ %s: %dms

and lots more ․ %s: %dms before

%d passing (%s)
%d failing

This was working perfectly before I added a new test. Literally nothing else has changed. I am on Mac Yosemite 10.10.2, and have just done a restart. What could be causing this?