A newer threat to domain owners is known as "snap" registration. Some registrars exist solely to monitor expiring domains and automatically register them the moment they become available.
The registrars then place generic, ad-heavy websites on the new domains, collecting revenue from misclicks and old visitors while hoping the previous owner will pay their ransom to recover the expired domain. (If you have ever visited an old bookmark only to find the site had been replaced with a strange banner-laden page, it was likely taken by a "snap" registrar.)
If you are the victim of a "snap" registrar, there are a couple things you can try that may allow you to recover the domain:
Don't visit the website! Most "snap" registrars take advantage of a loophole in ICANN policy that allows them to delete a domain within five days of registration for a full refund.
This activity, known as domain tasting, allows them to register hundreds of domains every month and never pay a cent in registration fees. If they "snap up" your domain, but their ad page receives no hits, they may delete the domain as undesirable within five days — at which point you can try to recreate it. However, even a handful of hits can mark a domain as "active", so it's important that you not visit the website until it is back under your control.