The bank sends you an email saying that it noticed a questionable transaction on your account and asks you to provide financial information in order to verify the account.
A pop-up ad says it needs your personal information to make sure you are the rightful owner of your online account.
The communiques may be legitimate, but they are most likely the work of fraudsters trying to trick you into providing sensitive data. The practice is called phishing and the Federal Trade Commission is holding a "twitter chat" on the topic at 2:00 p.m. Thursday.
To participate you must have a twitter account. For twitterers in the know, follow @FTC and use the hashtag #ChatSTC.
To take an amusing phishing-scam online quiz created by the FTC, click here. It's a little Nemo-like, but it gets the point across.