It's well within expectations that if you go to sleep with another person, you might wake up and see them the next day. However, when you fall asleep by yourself and wake up the next day with one or more additional bedmates, that tends to surprise people. Sometime during your slumber, someone else has crawled into your bed or futon and fallen asleep by your side.
The intruded upon individual tends to be the one to wake up first usually with some grand display of shock and/or surprise. They'll go through a "how did this happen?" mental routine, but typically won't be able to figure it out.
Their new bed partner could have any number of motives for this nighttime invasion, but has usually done it unintentionally. They were either drunk, really sleepy, confused, or honestly just made a mistake.
Often found in romcoms and usually invoked for laughs. Will almost always happen when there's a co-ed sleeping arrangement.
- Key Example - nearly every harem Anime series features at least one of these events sometime during the season