"Colossal blunder" refers to a very big mistake or error. It means someone has made a major, noticeable, and often disastrous misstep or decision. Full definition
Mulcair made a colossal blunder in ceding the progressive wing to the Liberals. (progressive-economics.ca)
Fortunately for those who really want to know the truth, Matthew made a colossal blunder later in his gospel which leaves no doubt at all as to which of the above possibilities is true. (religion.blogs.cnn.com)
Not so moderate was the statement by the leader of the Arkansas Baptists, printed in the Southwestern Journal of Theology in 1919: «The colossal Union Movement is a colossal blunder, but it threatens us Baptists unmistakably. (firstthings.com)