There are 3 instances of this:
Whenever the returned bytes data is not required, using the .call() function with non TRUSTED addresses opens the transaction to unnecessary gas griefing by return huge bytes data.
Note that this:
Is the same thing as this:
So in both cases, the bytes data is returned and copied to memory. Malicious target address can return huge bytes data to cause gas grief to the sender. 
Malicious target address can gas grief the sender making the sender waste more gas than necessary.
Short term: When returned data is not required, use a low level call:
Long Term: Consider using https://github.com/nomad-xyz/ExcessivelySafeCall
