[Rick Seaney is the CEO and cofounder of FareCompare, and columnist for Investopedia. The views expressed by columnists are those of the author and do not necessarily reflect the views of Investopedia.]

We’ve all seen it: a fantastic, almost impossibly cheap deal. Could it be for real? Sometimes, sure. Ultra-discounter Spirit used to be famous for its occasional $9 sale fares (it still has a $9 Fare Club). And just a couple of weeks ago, Wow Air was offering deals from Baltimore to London or Paris for $69.

So when is a deal not a deal? There are four things to watch out for. Finding one of them doesn’t necessarily mean the deal is no good at all, only that it might not be as good as you thought.

1. One-Way Deals

All the deals mentioned above are for travel in one direction, which is how a lot of airlines advertise prices – as one-way fares. The question left unanswered: Will you pay the same dirt-cheap price in both directions? Sometimes it works out that way, often it doesn’t.

Here’s an example for cheapest nonstop flights on a major U.S. airline for travel in October (found on the carrier’s site earlier this week):

  • New York to Berlin, $180
  • Berlin to New York, $491

The total price of the ticket is $671 – still a good deal for Europe, but not quite as good as $360 ($180 + $180).

2. Odd Times to Travel

I recently saw some good fares on…