Airlines can and do cancel tickets where the fare was a mistake. IIRC in the US it was held that an airline could do this if the mistake was an obvious mistake (I don't remember if that was a DOT decision, or if the case actually went to court)

BUT these ex-MPM fares were no mistake, they had been there for ~3years ever since QR joined the alliance. It's just that the local currency has been in steady decline with the result that the fares became very cheap to those of us outside Mozambique.

As regards an airline re-pricing - yes they do that if the fare has been held for more than a certain length of time (and I guess also when the fare has changed significantly). I was in the situation where I had phoned AA and paid on Friday (July 29). I was not ticketed and my credit card had not been charged when the fares went up on Monday. Yet I phoned on Tuesday and got it ticketed at the pre price rise fare.