The person making change is doing small arithmetic under someone else’s impatience.

Simple arithmetic gets harder with a queue behind you. Counting coins accurately while somebody sighs, while the next customer steps forward, while a manager watches the till, is a different task from doing the same sum alone. Pressure does not improve accuracy, and the person who makes the mistake under it will be the one held responsible.

Wait without performing your impatience: no sighing, no watch checking, no leaning. Say the amount clearly and let them count. Almost every job that looks simple contains a version of this, done while somebody stands over it.