In a dynamically language you need 100% test coverage, because misspelling a variable name turns into a runtime error.
On large projects this can be a problem. I worked on a Python server some years ago, and with every large program that supports exceptions, there's a catch block somewhere that silently eats the exceptions it doesn't process. Well, some modules were loaded dynamically, so my first spelling mistake (and second and third...) took quite a while to debug because things just didn't work, the code didn't get executed, but there were no errors. This also ate syntax errors, too, since that is also an exception. So if you edited the file, you had no idea if things actually still worked (or even compiled!) until you tested the functionality from the file.
On large projects this can be a problem. I worked on a Python server some years ago, and with every large program that supports exceptions, there's a catch block somewhere that silently eats the exceptions it doesn't process. Well, some modules were loaded dynamically, so my first spelling mistake (and second and third...) took quite a while to debug because things just didn't work, the code didn't get executed, but there were no errors. This also ate syntax errors, too, since that is also an exception. So if you edited the file, you had no idea if things actually still worked (or even compiled!) until you tested the functionality from the file.