Firstly it is a question what caused the price increase. Perhaps it is a fee or tax which is only added in the last step, possibly because it is jurisdiction specific? It could also be that there was just one seat left at the particular fare and someone was quicker - most airlines have in their T&C that the fare is only confirmed once it is paid.

A different story are error fare. Here the legal view is that the airline needs to honor them, if the appear reasonably possible (e.g. EUR 250 instead of EUR 300). But if they are absurd (e.g. EUR 250 instead of EUR 3000), they do not need to honor them.