Does Force Op Exist?

Generally speaking, no; there have been no widespread cases of a "force op" exploit existing.

However, there are a number of ways that a user could gain operator powers. As a server owner, you need to take care that unauthorised players can't become an operator:

1. Trust who you give operator powers. Any operator can make any other player an operator. If running a Bukkit server, you should ideally be using a permissions plugin with nobody - not even yourself - having operator powers.
2. You have a permissions plugin that is configured to give access to the /op command to someone who shouldn't have it.
   
3. Your server is running a malicious plugin that has a backdoor that provides operator powers.
   
   Do not install plugins from sources you do not trust. Only plugins downloaded from the Bukkit website should be trusted. Never install a plugin that has been given to you from outside the Bukkit website, this is a common tactic to get server owners to install malicious plugins.
4. The server is running in offline (aka "cracked") mode. In offline mode, anyone can login with any username they want. They can join with using the username of an operator and then give operator powers to anyone else and use operator commands.
   
5. An unauthorised individual has access to your Multicraft account, and thus access to your server's console. If this is the case, change your Multicraft password immediately.