This error can happen in one of two places, on the user side, or on the admin side.

When it happens on the admin side (wp-admin), you will find that you are unable to login.

Step 1: Connect to your blog using ftp and navigate to your wp-content/plugins/ directory and download all of your plugins to a folder on your desktop.
Step 3: Still in your FTP program, delete all plugin folders and php files in the plugins directory. You do not want anything at all left in this directory.
Step 4: Try logging into your blog. It should let you log in with no problems. Visit the plugins section of your blog. This should load with a lot of errors telling you that each plugin has been deactivated due to an error. This is ok.
Step 5: Using your ftp program you should upload all your plugins. Once uploaded you should activate each plugin one by one within wordpress and then visit the wp-admin/ login address to make sure that it will pass you through to your blog once logged in.
Step 6: When you activate a plugin and start getting this error again, you will know which one is to blame. Simply delete that plugin via ftp and start looking for an alternative plugin to use.

If these steps do not fix your problem, you should backup all your wordpress files and then delete them. You should them upload clean wordpress files for the latest version and try logging in with no themes or plugins uploaded. You can upload these later once you can login.

If you continue getting error, clear your browser cache, test logging in with other browsers.

If all else fails, contact your webhost or open a thread on the wordpress support forum.