I'm going to take a break on testability to talk about file paths - specifically long file paths, and the care one must take when dealing with them on Windows.
Now, while quite a lot of technologies are involved in development, for me (right now), the most important to mention when dealing with files are: Java, Subversion, Windows, and Tortoise SVN. I'll throw in one more - the filesystem (NTFS).
Using the command line is part of my nature, so when I get into the office in the morning one of the first things I do is open up a command prompt and 'svn update' my working copy.
Imagine my surprise one day when I couldn't update, because Subversion claimed one of its .svn/... files was corrupt or missing. Cleanup didn't help - it wasn't actually a Subversion problem. It was a file path problem. The svn FAQ actually has great information on this.
Enter the windows API maxium file path limitation of 260 characters. What's annoying is that it isn't ALWAYS the limit, in fact the unicode version of the API has a much more flexible limit (thousands of characters). Why then, does Windows Explorer, even in Windows 7, still use the API that has the 260 character limit? *
I deal with this issue again and again. I generally don't create file paths this long, but it doesn't matter - because I work as part of a team. I think it's reasonable for people to have descriptive, rich naming for their stuff - I just wish it wasn't so painful. Once files are in version control, then build tools, installers, and all kinds of things MIGHT have an issue depending on where the files are checked out and what file API they use. It's just not an issue I should have to deal with.
I don't want to worry about long file paths - this is a simple request. Of course, simple problems for computers aren't always simple. In fact, they usually aren't, but the fact that the API exists but that it isn't already the defacto, default API used by Windows itself frustrates me.
I want to be clear - this isn't a limitation of NTFS, Java, or Subversion. They are all quite happy to deal with long filepaths. It's the Windows file API. It's just easier than normal to get into bad situations when you have tools that will very happily (as they should!) create and deal with long filepaths, only to have your operating system tell you the file doesn't exist or can't be accessed.
* Probably backwards compatibility
That's my rant for the day.