In some contexts it makes sense to talk about errors in opposite directions canceling out but in others it does not as errors only accumulate. Suppose one person overestimates how much they'll enjoy having an iPad and buys one when they'd be better off without one, and another person underestimates how much they'll enjoy having an iPad and doesn't buy one when they'd be better off with one. Looking at the total number of iPads sold, these errors cancel out. But looking at total human welfare, the errors just add up - two people are each less happy than they could be, which is doubly bad. Similarly, if one person gets too much medical care and another gets too little, then they both lose, one from being overtreated and the other from being undertreated.

If you look at the market as a means of aggregating information (as in prediction markets) then errors can cancel out, but when you evaluate the market as a means of distributing products to people then errors just accumulate.