I got back from six weeks travel and fired up Eclipse (4.3 Kepler on Mac OS X). Not surprisingly, there were updates so I said ok to install them and restarted when prompted. Except instead of restarting it crashed :-( I tried restarting several more times but it kept crashing.
Rather than waste a lot of time figuring out what went wrong it was quicker to rename my old copy of Eclipse, download a new copy, and then import my plugins from the old install. (One of the nice things about Eclipse is that it's just a folder, and you can trivially have multiple installs.)
The only other thing I had to do was check which Java Eclipse was using (Eclipse > Preferences > Java > Installed JREs). I found it only listed an old version so I used the Search button to find the new one, made it the default, quit from Eclipse, and deleted the old version of Java to ensure nothing would use it.
You'd think that leaving a computer turned off would ensure that everything would work when you got back (barring hardware failures). But interrupting the modern day firehose of updates unfortunately often leads to problems. (Although in theory it shouldn't.)