The lowest fare on a search result is frequently a restricted class that removes several things travellers assume are standard. The restrictions vary between carriers and the pattern is consistent.
What is typically removed
The common list, though specifics differ by airline.
Seat selection, either entirely or for a fee, with assignment at check-in. For anybody travelling with others this frequently means being separated.
Carry-on baggage beyond a personal item, on some carriers, which is the restriction that catches people out most expensively since it is enforced at the gate.
Changes and cancellations, which are generally not permitted at all rather than permitted for a fee.
Upgrade eligibility and, on some carriers, reduced or no frequent flyer accrual.
And boarding priority, meaning last boarding group, which interacts badly with limited overhead space.
The gate check trap
The specific failure worth understanding.
On carriers restricting carry-on, arriving at the gate with a bag that does not fit the personal item allowance results in a fee that is substantially higher than checking it would have been at booking.
The fee structures are designed so that the cheapest path is deciding at booking, and the most expensive is deciding at the gate.
Which means the fare comparison should include baggage at the point of booking, not after.
When it is genuinely the right fare
It exists for a reason and suits some travel well.
Short trips with only a personal item.
Solo travel where seat separation does not matter.
Fixed dates with no realistic chance of change.
And routes where the price difference is substantial rather than marginal.
For that profile it is straightforwardly the correct choice and the restrictions cost nothing.
When it is a false economy
The situations where the cheaper fare ends up costing more.
Travelling with anybody you want to sit next to, since paying for seat selection frequently exceeds the fare difference.
Any bag beyond a personal item on a restricting carrier.
Any itinerary with a realistic chance of changing, since the change restriction is absolute.
Connections, since last boarding and no seat selection compound the risk of problems.
And anybody who needs a specific seat for a medical or accessibility reason, which is a separate conversation with the airline rather than a fare class question.
The comparison problem
Why this persists as a source of frustration.
Search results rank by price, which surfaces the restricted fare first, and the restrictions are disclosed at a later step.
Regulators in several jurisdictions have moved toward requiring clearer upfront disclosure of what is and is not included, with varying implementation.
The practical response is to compare total cost with the things you actually need added, which most booking tools now allow but do not do by default.
The status exception
Worth knowing because it changes the calculation entirely.
Frequent flyer status with a carrier frequently restores several of the removed benefits — seat selection, boarding priority, baggage allowance — even on restricted fares.
Which means the same fare class is a very different product depending on who is buying it, and status holders can book the cheapest fare without most of the consequences.
Certain co-branded credit cards confer similar benefits, particularly baggage allowance, which for a frequent flyer on one carrier is frequently the largest practical benefit of holding the card.
What I actually do
Price the fare with the things I need included, rather than comparing headline numbers.
Check the specific carrier's carry-on policy for that fare class, since it is the restriction with the largest downside.
Take the restricted fare for short solo trips with fixed dates, which is most of my domestic travel.
And avoid it entirely on anything with a connection or anybody travelling with me, where the accumulated restrictions reliably cost more than the saving.
The disruption position
The consequence that matters most when things go wrong.
When a flight is cancelled or delayed, airlines rebook passengers, and the order in which they do so is influenced by fare class and status.
A restricted fare passenger is generally reaccommodated after others, which on a full day of disruption can mean a substantially later flight.
Statutory protections requiring rerouting apply regardless of fare in some jurisdictions, and the practical experience of exercising them still differs.
Which is a risk rather than a fee, and it is worth weighing for anything time-sensitive.
The personal item, measured
The specific dimension worth checking since it is where the enforcement happens.
Personal item allowances are stated in dimensions rather than by description, and they vary between carriers by enough to matter.
Sizers at the gate are used inconsistently and are used, and a bag that fits one carrier's allowance may not fit another's.
Measuring your own bag once and noting the dimensions removes the guessing entirely, and a soft bag that compresses is considerably more forgiving than a rigid one.
Booking directly
Worth noting that fare classes and their restrictions are described more clearly on the airline's own site than on aggregators, where the comparison is by price and the conditions appear late.