A mix-up is a mistake or confusion that happens when things get jumbled together or switched around. When your teacher accidentally hands back someone else's graded paper to you, that's a mix-up. When you show up for soccer practice on Tuesday but practice was actually scheduled for Wednesday, that's a mix-up in the schedule.

Mix-ups often happen with communication. Your mom might tell you to meet her at the south entrance of the library, but you heard north entrance, so you're both waiting in different places. That's a classic mix-up. Or imagine two students named Alex Johnson and Alex Jones: if the school sends one Alex's report card home with the other Alex, that's a mix-up that needs fixing.

The word suggests an honest mistake rather than something done on purpose. A mix-up is usually fixable once everyone realizes what went wrong. You might say “there must have been some mix-up” when you're trying to figure out why things didn't go as planned. The key is catching the confusion and sorting it out before it causes bigger problems.