Cold call
Calling a raise having put no money in yet. Often the start of a leak — flatting hands that should raise or fold.
Say UTG opens to three big blinds with A-K, it folds to the button, and the button cold calls 8-7 suited. The blinds fold, so it is heads-up and in position for one raise: a cheap flop, deep implied odds, and a hand that can stack A-K when it flops two pair or a straight. That is cold calling done right. You did not open, you owe nothing to the pot yet, and you are using position to see a flop cheap with a hand that flops well.
The cost is what the call does to your range and your stack. A cold call leaves the opener's raise as the only aggression in the hand, so the pot stays small next to what a raise would have built, and you are always reacting to someone else's sizing instead of setting your own. It also invites the players behind you back in. A cold call from early or middle position wakes up the button and the blinds, and your suited connector ends up in a four-way pot where most of its edge is gone.
The beginner leak is cold calling a hand that should raise or fold — K-9 offsuit from the hijack against an UTG open, with the cutoff, button, and blinds still to act. Too weak to 3-bet, too curious to fold, so it flats, drags the field along, and plays a dominated hand in a bloated pot, often out of position when the button calls too. Raise the hands worth raising, fold the junk, and cold call only when a call beats both.