Calling 4th Street with a 3 Flush
The mistake we're talking about here is when you're dealt a pretty 3 flush as your first three cards, only to get a blank on 4th street. Many players will call with the 3 flush, but in fact your odds against making the flush by the river is 8.5:1 --Pretty ugly odds.
A 3 flush on 4th street is not an automatic fold, but you'll need other compelling reasons to stay with the hand. A pair or 3 overcards to the entire board, or a 3 straight to go along with your 3 flush are examples of reasons you may choose to stay with the hand.
If your hand is only playable as a flush and you only have 3 of your suit on 4th, then the all knowing pot odds probably say fold. And we all know they're right in the long run!
This strategy follows the axiom that a good player looks for reasons to fold, not reasons to hang on to a marginal hand.
Exception: In a crazy game with hi antes and everyone seeing 4th street, you may very well have correct odds to call! Remember 8.5:1! Wild games with high antes allow for marginal hands to become correct pot odd calls at times!