Ultra-Leftists are typically other communists who are not rooting their actions and methods in material reality. This false consciousness held by ultra-leftists leads to incorrect theoretical and practical conclusions. Conclusions that can endanger the movement itself. Practical concessions need to be made if the reality of the situation demands it. Ultra-Leftism, an Infintile Disorder by Lenin, covers this topic in around 90 pages. Worth reading if you haven't. Marxism is a living science and, as such, can adapt and evolve as conditions change. To not do so would lead to dogmatism and inflexibility, which undermine the communist movement.