There was a SO question with bad indented code. I edited it and it seems like (maybe, I'm not sure) I deleted a part of the question when edited it (I've cut, edited and pasted).
So how do I see the old version of the answer?
Let's assume you've made a mistake when editing the question. First, you can go to the revision history of a question to see the various revisions made. An easy way to get there is to simply click on the timestamp of the "edited" notice of a post.
Within the revision list you can look at the various changes made. Of course you can correct any errors you notice with a simple edit like you did before. But in case your mistake is too big (or too complex) to correct, you can always roll back to a previous revision. To do so, simply click on the "rollback" link of the revision you want to step back to.
That should do the trick. Once everything is back to normal, you can then carefully reapply any edits you really wanted to make. So in summary, be careful when editing, but if you ever do screw up, don't worry. You can always go back.