1) Incorrect social-political orientation towards proletariat at the time when computer revolution was dawning. See Jean van Heijenoort's 1948 analysis. This is a crisis (deep fault) of Marxist theory that has led to many organizational problems.
2) Trotsky was from the start a centrist, e.g. arguing for a union between Bolsheviks and Mensheviks, in 1903 Congress. In 1930's, he advocated "the French turn", i.e. entry into a socialist party, first in France, later in the United States, in order to increase membership.
3) This organizational mistake was compounded by leadership of Pablo, with policy of "entryism sui generis" into Stalinist parties. Moreover, it has led to followers of Trotsky betraying his most important contribution to social-political theory, his analysis of the USSR as a transitional state. For example, followers of Cliff argue that USSR is "state capitalist" and hence Britain should intervene in Chechnya against Russia.
4) The IV International has been used by the ruling imperialist powers in their struggle against the USSR, since WWII. First, it started with IV International failing to see Tito as another instance of Stalinism (for a more realistic analysis, see Milovan Djilas' "The New Class"). Then, the (British) Trotskyists (followers of Tony Cliff) support the Chechen rebels against the Russian state. They have abandoned the social-political analysis of USSR, the best outcome of Trotsky's fight against Stalin.