|laziness, impatience, and hubris|
Firstly, the author of a reply (note) can edit the reply, as can the poster of certain root node types (meditations I believe, and code, maybe others). If you want to get something changed in a root node, you can use an editor request, or use NTC if you are level >=6.
An alternative is to reply to your own root node, and post the update/retraction/apology into the reply.
When it comes to editing your own posts, it is a matter of etiquette. If you are changing the content of what you have said, it is customary to do one of the following:
Append a section headed Update:
Use a combination of strong and strikethru to show what you have changed.
I do not believe it is worthwhile showing trivial corrections such as typos and spelling mistakes. Corrections to code however should be highlighted in case someone has already downloaded it.
Hope this helps,