Two different things are described as travel deals. Promotional sales, which are planned, and pricing errors, which are not. They behave very differently.
How errors occur
Fares are constructed from base fares, taxes, surcharges and currency conversions, distributed through several systems.
Errors arise from a missing fuel surcharge, a currency conversion applied incorrectly, a tax component omitted, or a filing mistake in a distribution system.
The result is a fare substantially below any commercially plausible price, occasionally by an order of magnitude.
These are found quickly by people monitoring for them and circulate rapidly, which is why they are generally corrected within hours.
Whether they are honoured
The question that determines whether booking one is worthwhile.
Practice varies by carrier, by jurisdiction and by how obviously erroneous the fare was.
Some airlines honour them, treating it as a marketing cost. Some cancel and refund. Some offer a voucher or a discounted rebooking as a compromise.
Consumer protection rules in some jurisdictions constrain the ability to cancel a confirmed booking, and in others a clear pricing error may render the contract voidable.
Which means the outcome is genuinely uncertain and depends on where the booking was made and with whom.
What to do if you book one
The conventional advice among people who track these, and it is sound.
Do not book connecting arrangements — accommodation, onward flights, activities — until the ticket is confirmed and has been issued for a period.
Do not contact the airline to ask whether it is genuine, which draws attention to the booking.
Pay with a card that provides protection.
And be prepared for cancellation, treating the booking as an option rather than a plan.
The financial risk is generally limited to the fare and the inconvenience, which is why the practice persists.
Promotional sales, which are different
Planned reductions with a commercial purpose.
These are genuine and are targeted — specific routes, specific travel periods, generally off-peak, with restrictive conditions.
The saving is real and the restrictions are where the cost sits: fixed dates, no changes, limited baggage, and travel windows that exclude the periods people want.
Which means the headline fare is available for travel at times most people cannot use, and the fare for a usable date is considerably higher.
The dynamic packaging alternative
Worth knowing since it frequently beats separate booking.
Flight and accommodation booked together as a package are sometimes cheaper than the same components separately, because the operator can discount one against the other and because package rates are negotiated.
The other advantage is regulatory. Packages in many jurisdictions carry statutory protections covering insolvency and significant changes, which separate bookings do not.
That protection is substantial and is discussed elsewhere, and it is the strongest argument for the package format.
The mistake fares that are not
Worth distinguishing since several categories look similar.
Fares excluding a mandatory extra, where the total after unavoidable additions is unremarkable.
Fares on routings that require multiple connections and eighteen hours, which are genuinely cheap and genuinely unpleasant.
And fares in a currency where the conversion looks favourable and the payment currency differs, which is a real cost rather than a saving.
The alerts question
Since the deals are found by monitoring rather than searching.
Several services publish fare alerts, some free and some subscription.
The subscription ones are worthwhile only for somebody genuinely flexible about destination and date, since that is the condition under which the alerts are actionable.
For anybody who needs to be in a specific place on specific dates, the alerts are entertainment rather than a tool.
The honest position
Error fares are real, occasionally spectacular, and unreliable.
Promotional fares are real, restricted, and predictable in their timing.
And flexibility is the currency that converts either into an actual trip, which is why the people who benefit most from both are the ones with the least fixed plans.
The twenty-four hour rule
A consumer protection worth knowing where it applies.
Some jurisdictions require airlines to allow cancellation without penalty within a short period after booking, provided the departure is sufficiently far ahead.
Which converts a speculative booking into a genuinely low-risk one, since it can be released the following day if plans do not come together.
Where such a rule applies it is a substantial benefit and it is not advertised, and where it does not the booking is committed immediately.
Booking directly versus through an agent
A distinction that determines what happens when something goes wrong.
A fare booked directly with an airline means one party to deal with for changes, cancellations and disruption.
A fare booked through an online agent frequently means the airline directs you to the agent and the agent is slow, which during widespread disruption is a genuine problem.
The price difference is sometimes meaningful and the difference in recourse frequently is, which is worth weighting for anything where the trip matters.