When attempting to use PayPal, you may encounter an error "Sorry - your last action could not be completed".
This is the due to the way your browser deals with cookies.
You can easily solve this, as follows:
For Internet Explorer:
Go to Tools -> Internet Options, click on the Privacy tab
Click the Advanced button, and then check the checkbox that says "Override automatic cookie handling".
Then click OK to close the "Advanced Privacy Settings" dialog box, and OK to close the Internet Options dialog box.
For FireFox 3.5:
Go to Tools > Options click on Privacy tab
In History Firefox will: Select Use custom settings for history
Ensure the Accept third-party cookies box is checked
(No restart is required)
If you are running an older version, then the menus may differ, but look for a setting that accepts 3rd-party cookies.
A great way to test that it works, is to try it with this link, and click on the Donate button.