Whew, the data loss in the last post actually wasn't that bad because the folder was version controlled anyway (and stored on another server). I thought only using dropbox for working copies would keep me safe.
This comes back to the symlink semantics, but it can bite you in strange ways. Here's how you can really screw yourself. First, let's say you've got a working copy that stores all your notes and other reference text files that you search often:
mv foobar ~/dropbox/foobar
Let's further suppose that you are on a Mac or Linux client and foobar contains symbolic links that point to other scattered text files elsewhere in your personal archives that you may want to have at hand (for grepping).
Next, you want to make a major change to foobar, so you back it up, just to be safe:
cp -a ~/dropbox/foobar ~/dropbox/foobar_bak
A while later you come across this backup (on a different dropbox-linked machine) and delete it:
rm -rf foobar_bak
BAM! That's it, you just screwed yourself.
How? When Dropbox went back to your original Mac/Linux machine to delete foobar_bak, it decided to FOLLOW THE SYMLINKS. That's right, it deleted the original files. Even though the original foobar directory is still there, its links are now broken of course.
The whole point of these links was organizational. They pointed all over the place. Even if you have backups you now have to track them down and restore those scattered files. (Which is what I just spent my time doing.) I guess I'm a synchronization masochist because I seem to ask for this kind of punishment.
Bottom line. If you're a power user, avoid Dropbox or keep your usage very light, e.g. stick an office document in there every once in a while. Or be ready to suffer.