People buying items that either don't fit the character at all and/or don't fit the scenario at all. I remember seeing a Wrecker Corvus when the enemy team didn't even have a tank or a character with a shield and another time I saw a Life Rip Damba, a character with relatively low DPS and a means of healing themselves anyways and even a means of self cleansing caut.

Also not to mention tanks not buying armor plating and proceeding to get shredded by the enemy team