What's with the advice about interrupts and undebounced signals? What does it mean that a flip-flop gets confused? What kind of undesired operation could that

What's with the advice about interrupts and undebounced signals? What does it mean that a flip-flop gets confused? What kind of undesired operation could that cause? Because, quite honestly, if connecting directly means occasional transient failures, then having less hardware is a tempting tradeoff on small PCBs.