Socially Ambivalent Labour Time XII: Capital volume III, chapters 5 & 10

In chapter five of volume III, Engels made a blunder by referring to socially necessary labour time as necessary labour time. Presumably, the error originated in Marx's notes and Engels didn't notice and correct it:

If it is the necessary labour-time which determines the value of commodities, instead of all the labour-time contained in them, so it is the capital which realises this determination and, at the same time, continually reduces the labour-time socially necessary to produce a given commodity.

This may seem like nitpicking except that, for Marx, necessary labour time is defined as the time required to recover the cost of labour power and is explicitly differentiated from surplus labour time. The error was not introduced by the translation, the original German also uses notwendige Arbeitzeit where gesellschaftlich notwendige Arbeitzeit would be required by context:

Wenn der Wert der Waren bestimmt ist durch die in ihnen enthaltne notwendige Arbeitszeit, nicht durch die überhaupt in ihnen enthaltne Arbeitszeit, so ist es das Kapital, das diese Bestimmung erst realisiert und zugleich fortwährend die zur Produktion einer Ware gesellschaftlich notwendige Arbeitszeit verkürzt.

We can figure out what Marx/Engels means here but maybe it would have been better to call socially necessary labour-time something more distinctive from necessary labour time?

Chapter ten presents an analysis of how the accidents of supply and demand can result in a price for a commodity above or below its market-value, as determined by socially necessary labour time. It is worth quoting in full:

In the case of supply and demand, however, the supply is equal to the sum of sellers, or producers, of a certain kind of commodity, and the demand equals the sum of buyers or consumers (both productive and individual) of the same kind of commodity. The sums react on one another as units, as aggregate forces. The individual counts here only as part of a social force, as an atom of the mass, and it is in this form that competition brings out the social character of production and consumption.

The side of competition which happens for the moment to be weaker is also the side in which the individual acts independently of, and often directly against, the mass of his competitors, and precisely in this manner is the dependence of one upon the other impressed upon them, while the stronger side acts always more or less as a united whole against its antagonist. If the demand for this particular kind of commodity is greater than the supply, one buyer outbids another — within certain limits — and so raises the price of the commodity for all of them above the market-value, while on the other hand the sellers unite in trying to sell at a high market-price. If, conversely, the supply exceeds the demand, one begins to dispose of his goods at a cheaper rate and the others must follow, while the buyers unite in their efforts to depress the market-price as much as possible below the market-value. The common interest is appreciated by each only so long as he gains more by it than without it. And unity of action ceases the moment one or the other side becomes the weaker, when each tries to extricate himself on his own as advantageously as he possibly can. Again, if one produces more cheaply and can sell more goods, thus possessing himself of a greater place in the market by selling below the current market-price, or market-value, he will do so, and will thereby begin a movement which gradually compels the others to introduce the cheaper mode of production, and one which reduces the socially necessary labour to a new, and lower, level. If one side has the advantage, all belonging to it gain. It is as though they exerted their common monopoly. If one side is weaker, then one may try on his own hook to become the stronger (for instance, one who works with lower costs of production), or at least to get off as lightly as possible, and in such cases each for himself and the devil take the hindmost, although his actions affect not only himself, but also all his boon companions.  

The compulsion to reduce socially necessary labour by introducing cheaper methods of production thus engages the same competitive pressures as the discrepancy between supply and demand. The implication is that the pressure to economize production is especially intense in industries that already confront a surplus of supply over demand. I don't know what the empirical evidence for (or against) this is but it would be interesting to find out.