Every long trip contains a week that nobody photographs. Usually it contains several. Knowing what they look like in advance is not pessimism, it is the difference between a bad week and a trip that ends early.
You will be ill
Not might. On a long trip through South or Southeast Asia, Africa or Latin America, some form of stomach illness is close to universal, and it is usually three unpleasant days rather than anything serious.
What it looks like: a day of feeling wrong, a bad night, a day of not being able to leave the room, and then improvement.
What to do: rehydration salts and more fluid than you want. Rest. Plain food when you can face it. Do not travel on the worst day if you can avoid it, because a six-hour bus is the wrong place to be.
When it is not routine: blood, high fever, severe abdominal pain, or symptoms lasting more than a few days. Those need a doctor, and in most of the world a private clinic is cheap, quick and good. See the health guide.
Build slack for it. Anyone travelling for three months who has not allowed for a week of being unwell has planned badly.
Laundry is a constant low-level problem
Everywhere has a solution and it is different everywhere: a wash-by-weight service in Asia, a laundrette in Europe, a sink and a universal plug elsewhere.
Practical: carry a universal sink plug and a small tube of travel wash, and choose clothes that dry overnight. Cotton in humidity does not.
The recurring failure is handing everything in and then needing to leave before it comes back. Ask when it will be ready and add a day.
Some accommodation will be bad
You will arrive somewhere that is nothing like the photographs, or that has a nightclub below it, or that smells.
One night in a bad room is survivable. The mistake is booking a week and then enduring it out of inertia. Leave. The money is gone either way and the alternative is a week of misery.
Look at the room before you pay where the culture allows, which is most of the world outside the online-booking system.
Something will be stolen
Statistically, on a long trip, probably. Usually a phone or a bag, usually through inattention rather than force.
What limits the damage: two cards kept separately, cloud backups running, a photograph of your passport stored offline and online, and not keeping everything in one bag.
If a passport goes: report it to the local police for a written report, then contact your embassy. An emergency travel document takes days rather than weeks in most places, and the process is much faster if you have a copy of the original and a spare passport photograph.
Insurance claims require the police report, filed usually within 24 hours. See the insurance guide.
The admin never stops
Visas two countries ahead. An insurance renewal. A card blocked because the bank saw an unusual country. A passport running out of pages. Tax at home. A parcel you cannot receive because you have no address. A subscription you cannot cancel because the site wants an SMS to a number you no longer use.
Set aside an hour a week. It does not go away and it compounds when ignored.
Two-factor authentication by SMS to a home number is the single most common way people lock themselves out of their own bank, email and everything else. Move to an authenticator app before you leave.
The day everything goes wrong at once
It happens on most long trips: a missed connection, plus rain, plus being ill, plus somewhere that turns out to be closed, plus a card that will not work.
The only useful advice is procedural. Stop moving. Get a room, even an expensive one. Eat something. Deal with one problem at a time, starting with money and shelter. Do not make a decision about the whole trip on that day, because the decision you make will be to go home and it will not be the right one on Thursday.
You will get tired of it
Somewhere between month two and month four, the novelty stops. Another old town, another market, another bus. This is normal, it is not a sign the trip has failed, and it does not mean you should go home.
The response is not more stimulation, it is less. Stay somewhere for three weeks. Do something with a routine in it. Stop sightseeing entirely for a fortnight. The appetite comes back, and it comes back faster if you stop forcing it.
Coming home is its own problem
Covered in more detail elsewhere, but worth flagging here: the return is frequently harder than any week on the road, and it is much easier if there is something to arrive into rather than an empty date.
Why any of this is worth it
Because the good version of a long trip is not the absence of these things — it is the discovery that they are all survivable, that you can be ill in a country whose language you do not speak and be fine, and that a plan falling apart is a normal Tuesday rather than a catastrophe.
That is the thing people actually take home, and it is not visible in any photograph.