So, although I do appreciate the many fine aspects of my Mac, there are nevertheless still some annoying things about it. Today I came across a particular problem affecting Time Machine. It seems that Time Machine, when initializing a new drive, requires at least 20% or so of additional free space. Makes sense. However, it doesn't require that later on - leading to weird behavior as described below.
So, OS X informed me that my drive was too small by like 900MB for the backup, and that I'd have to find a new drive for Time Machine. So, I deleted some files (more than 900MB), and tried to back up again. Still no dice. Figuring it was trying to do an incremental backup, I decided to wipe the drive so it would only have one snapshot on it. Doing that resulted in a serious issue - a drive that had started with more than 20% free space now only had like 5% or less (my backup and regular drive are the same size). So, long story short, I was now ever farther from ever getting Time Machine to play nice with my drive. I ended up having to move a bunch of files to an external disk, run Time Machine, then move them all back over after the new drive was initialized. That's a huge hassle, and I'm not sure what I would have done if I didn't have another drive I could put the files on in between.