To fall into/avoid the trap of doing something
Definition 1 of 2
Pro Tip 1/3Unintended Consequences
Understanding that the phrase often implies a negative outcome that wasn't planned or foreseen. 
He fell into the trap of gambling, hoping to win big.
Pro Tip 2/3Subtle Warning
Recognize the phrase as a caution against common mistakes or poor decisions in various scenarios. 
Don't fall into the trap of skipping classes; it harms your education.
Pro Tip 3/3Negative Patterns
The phrase can indicate the beginning of a bad habit or continuing unhelpful behavior. 
She fell into the trap of lying to her friends.