Can I believe this error?
Recently, I typed in a wrong address and was taken to a strange website. Suddenly a message flashed up on the screen, my MacBook chimed, and a message popped up that said: “Do not shut down this computer. Do not make any purchases. Call toll free number”. I contacted my bank, which said it would check for possible fraud, but I am scared to use my credit card now. Danil Dyachenko
This is a phishing attempt. Phishing is when criminals attempt to trick you into giving away personal information, such as bank account details and passwords. In this case they are trying to scare you into thinking that you have already been hacked, so that they can go ahead and hack you. Often, criminals will register domain names that are simple typos of legitimate popular sites and wait for unsuspecting users to visit by accident. The messages are hoaxes, but the links on the web page may try to download malware if you click them. This is unlikely to work against a Mac, since almost all of this sort of malware targets Windows, not OS X, but calling the number is definitely a bad idea.
To dismiss a persistent message, force Safari to quit (press å+ç+oe, select Safari and then click Force Quit). Other than that, there is no need to worry. Phishing sites rely on spooking you into acting rashly, but they are mostly just bluster.