When you make a bad play which you would never have made if you'd noticed, figured out, or remembered some available piece of information, that's a blunder.
In general, blunders are unsatisfying to all of the players in a game. It would be unsatisfying to lose a game of Chess simply because you failed to notice that your king was about to be captured, or because you made a move that opened your king up to capture. This is the rationale behind the concept of check.
As a designer, you cannot completely eliminate blunders from your game, but you can reduce them, by increasing the game's clarity. If a particular type of blunder occurs over and over again in your game, that indicates a problem with the rule-set or the components, not with the players.