Every trade has a maker and a taker. The taker chose the moment, so the taker pays: half the bid-ask-spread relative to the midpoint, plus a take fee on most maker-taker venues.
Taking is not a mistake. If your edge depends on being in the position now, paying a cent to be certain beats resting for an hour and missing the move. The error is taking out of impatience when the trade had no urgency.
Example: a 2,000-share buy on a 19.98 / 20.02 market costs 2 cents against the mid, or $40, plus roughly $0.0030 per share in take fees, about $6. Resting at 19.98 instead would have earned perhaps $0.0020 per share in rebate, a $92 swing per round trip — real money for an intraday strategy, noise for a six-month hold.
Related: resting-order, maker-taker, bid-ask-spread, effective-spread