The good news is you can restore your system to Mountain Lion and avoid all of Mavericks but it is not a straight forward process if you didn't create a system backup.
Were you running Time Machine before your upgrade to Mavericks?
I would also make a complete backup of your current system just in case you missed something.
For that use SuperDuper or Carbon Copy Cloner to an external Hard Drive, duplicate it to make sure.
They both make a complete or partial back up of your entire System. The backup can be a compressed mountable .dmg file if you want. This makes a file that is smaller than your Hard Drive and can be stored for opening later or recreating elsewhere, and is an essential part of a Mac admins toolkit.
The worry with changing any system is that you may need to switch back or get at something that you left behind.
A good idea is to step across to new systems using an alternative hard drive just for that version.
I am doing that at the moment with Maverick and did it both with Lion and Mountain Lion.
With Lion I rapidly determined how bad it was and simply dumped it.
Well, I managed to figure out what causes this issue. Basically, Pages is attempting to autocorrect something as I type, but it also allows me to input a command before it applies the autocorrection. My command is usually opt+delete or cmd+delete to delete the typo. The situation unfolds as follows:
The catch is that the command I told it to run has already deleted <wrong>, so there is now completely different text in <wrong>'s position. Pages ends up replacing whatever is in its place. This does not happen with Fn+delete or just plain delete, since these don't appear to count as "appropriate input". I don't know what all qualifies as "appropriate input" as I haven't really tested that. To note, if Pages has multiple options for replacement words, then using opt+ or cmd+delete will behave as expected. I presume this is because it doesn't know what exactly to replace <wrong> with, so it aborts correction.
Anyway, turning off auto-correct spelling completely stops this.