You are looking for a roommate and you post an online ad to try and woo someone with similar interests. You write a blurb about what you're looking for in a flat mate, including hobbies and musical tastes, and post it.
A week later, you get a message from someone who, surprisingly enough, is exactly what you're looking for. She says she's moving to town for a job, and therefore money won't be an issue. She's also clean and courteous. Not only that, you guys have a ton in common! After a few weeks of chatting, you two agree you'll make a good team and you are excited about it.
She says she's arriving on the 9th, but she has no problem playing the full month's rent. She mails you the check--but whoops! She wrote down the wrong amount. Instead of the $450 you asked for, you get an extra zero in there: $4,500! You let her know about the mistake. She tells you to cash it so you can pay the rent, but she asks you to wire her back the extra money, so she isn't completely broke. You do as she asks, only to find that just a few days later, the check bounced, and you're out thousands of dollars.
How to avoid: either ask for a new check or ask for her to wire you the money. The other option is to cash the check and wait for it to clear before sending her all your money, but that could be valuable time you could be using to find a new roommate who isn't trying to take you for all you're worth.