The difficulty of identifying at any given time, statically (like an instantaneous photographic image) the structure. Politics in fact is at any given time the reflection of the tendencies of development in the structure, but it is not necessarily the case that these tendencies must be realized, A structural phase can be concretely studied and analysed only after it has gone through its whole process of development, and not during the process itself, except hypothetically and with the explicit proviso that one is dealing with hypotheses.
From this it can be deduced that a particular political act may have been an error of calculation on the part of the leaders of the dominant classes, an error which historical development, through the parliamentary and governmental 'crises' of the ruling classes, then corrects and goes beyond. Mechanical historical materialism does not allow for the possibility of error, but assumes that every political act is determined, immediately, by the structure, and therefore as a real and permanent (in the sense of achieved) modification of the structure. The principle of 'error' is a complex one: one may be dealing with an individual impulse based on mistaken calculations or equally it may be a manifestation of the attempts of specific groups or sects to take over hegemony within the directive grouping, attempts which may well be unsuccessful.
Forge, D., 2001, The Antonio Gramsci Reader: Selected Writings 1916-1935. p.191.