I dropped by one of my Google Code projects, a trivial little jQuery
placeholder functionality shim called place5, and was quite surprised to see, on the Source tab, the message "Error retrieving directory contents". Now, I have backups (and you do too, of anything you host externally, right?), so I wasn't panicked or anything, but I was still...jarred.
Some searching around revealed that people get this when they've changed their source control system (e.g., from mercurial to git, or subversion to mercurial, etc.). I hadn't changed my source control system, but looking at the Source tab again I saw that it was showing an empty git tree (with the error), and I thought: "Was I using git for this?"
Answer: No, I wasn't. I'd been using subversion. So what the...? So off to Administer, and under Source | Repository Type it said "default" (which appears to [now] be git). Setting it back to subversion corrected the error and restored source browsing.
I mention this for two reasons: Firstly, I figure I won't be the only one, so I thought I'd write this down in case someone runs into the same problem and finds this post. But secondly, it highlights a general principle: It's very useful to have your settings system have the concept of a "default" choice, so if you update defaults, people start seeing the new thing. But when you update a default that's going to break things for people, it's pretty important to change the setting for anyone using "default" to the new, specific choice for the old default. Because breaking things unnecessarily is doubleplus ungood, ya know?