I deleted one of my previous posts, but it still can be seen. Is there a problem?
You can still see the post, and that is by design; you might want to undelete it after a while if you found out how to improve it. Most other users cannot see it; you can test this yourself by opening the post in a private browser window (or a different browser where you're not logged in). It should look something like this:
Other users with at least 10k reputation can also see it if they have a link to it. And finally ♦ moderators and staff can see all your deleted posts. For example, if a user decides to rage quit, deleting valuable content, they can step in and rescue the content.
For more information, check out the FAQ on Meta Stack Exchange.