Look Behind You

it's R̴̼͓͌̈́͜͝I̸̝̪͔͛͒̽G̸̟͕̾̓̕H̸͎̞̐̈́͊͜T̸̢̡́́̕ B̸̡͎̪͒͒E̵̙̫͎̽̈́H̴̞͔̒̓̕I̵͙͓̝͒̔͝N̵̢̟̪͋̚͠D̴͇̺̠̀͆̒ Y̵͉̙̙͆̀̚Ò̵͉͙̼͆̐U̴̼͓̟̾͋͆

When a character, often another one's enemy, tells them to look back at something behind them. Even though this UnTrash has been played many times before over the years, there are only two ways this UnTrash can end its play: either the listener listens to the speaker before they make their getaway (situation A) or the listener refuses to fall for "that trick" even though there really is something behind them (situation B). If situation A is true, the remaining character will feel stupid for falling for such a common, basic trick before heading off after the other one. If situation B is true, then whatever is behind the listener is likely going to attack them when it gets the chance.