Any questions about practicalities of Wiki, put 'em in here:
(You may also want to try the WikiMorals pages for etiquette/ethics type questions.)
No, Wiki will warn you - then you can click Back, copy what you wrote, go to the original page, click EditText again, and paste in your changes. Phew! (Simpler than it sounds.)
We could know their IP address so theoretically they could be traced. However, Wiki works on the principle that there's no real point messing it up: it happens very occasionally and the participants experience some collective grief before picking up and carrying on.
Some Wikis keep all previous versions. Some keep none. This one keeps one previous version but no more. This makes what we build here very fragile and hence more precious?
(Note from AndyAtGuiltyExpression - I'll try to remember to take backups every now and then but don't count on it!)