There was a flaw, which allowed users acting within the normal limits of the United site to book incorrectly priced flights. They may or may not have had the intent to obtain incorrectly-priced flights. They may or may not have known the prices were due to a glitch, rather than a deep discount of sorts.
United will likely cancel all reservations, as they will be easy to identify, and haven't been ticketed yet. They will not needlessly go after users who received incorrectly-priced flights, pursuing legal suits of questionable merit and fueling a PR disaster that has at its root, questionable software practices at the company.