Game Over is when a minigame ends. In games with no victory-system (goes endlessly until you lose and in some minigames, it gets harder the more you play), an end screen will appear to the player, typed Game Over and the reason of the game ended. The reason will depends, since it can be change due the form that the game ended.
Also a little sound will play when the Game Over screen appears to the player.
When Pou gets sandwiched on platforms, leaving no room to fall.
Time's up!
Happens when minigames have a timer. The stopwatch time (consequently the color) may vary. When reaching the end of the time, the message "Time's up!" will appear on the final screen.
There are some games with different gameplay modes. Consequently, the endgame messages of these games are unique. Messages can be "You win!, as well as "You lose!". These games are the games in which it is possible to win/lose a match.
When the opposing Account beats you. In online games, the message is a little different than "You lose!". The message for these games is the opponent's name + "wins!" instead (ex: Pouwins!) Read more about PvP mode.
There are two minigames that don't have game-end screens: Pool and Goal. These minigames are from Outside. When the player fails, the score number simply resets all over again, returning to zero, without an end screen.