Bright idea: Check your plugins!
I’d been struggling valiantly with a blog that refused to save properly without giving an error (plug: www.ryersonformulasae.com, check it out!) until earlier today I decided to try modifying the settings until something changed. After deactivating the first plugin–which happened to be of my own creation–the problem was solved.
So now I know the solution, and would like to share it with others: if your WordPress installation is messed up, try deactivating your plugins one at a time and maybe your problem will work itself out.