Maybe it was the threat of another sober night sipping bottles of Bavaria, Iran's famously alcohol-free lager.
Or perhaps it was the mechanical breakdowns and hours of mind and bottom-numbing coach travel that threatened to bring the dream crashing down.
Whatever the final straw, on Sunday night it was reported that the 38 passengers aboard the OzBus, a 1960s-style charabanc trip from London to Australia, were threatening to mutiny.
Just three weeks into the three-month odyssey in which they were due to travel from London to Sydney via Turkey, India and Indonesia, a desperate email from one passenger to her family announced that, following yet another technical hitch with their Latvian-registered jalopy, consigning them to an extra unscheduled night in Tehran, the mood on board had blackened.
"We are currently stuck in this hellhole waiting for a replacement bus ... mutiny is in the air and everyone is so angry it is unbelievable," complained Lucy Allen, 22. "The only food supplies we've been able to buy since Istanbul is from petrol stations," she added.
While Tehran can prove challenging to Western tourists, especially women who must be well-covered if they are not to draw unflattering looks, and due to Islamic rules on alcohol which will hardly be welcomed by a busload of backpackers, "hellhole" still seems a little strong.
According to the tour organiser, Mark Creasey, things were not so bad. It was true the bus had suffered its share of travails on the way – a broken exhaust, a damaged wing mirror and a reversal into an unyielding tree – but morale was still high and they were now en route to Esfahan, a World Heritage Site and, more prosaically, the location for Iran's controversial uranium conversion facility.
Some of the travel blogs published by those on board would seem to back him up – containing the sort of gleeful tales of bar and tent hopping that might be expected from young people on the road trip of a lifetime.
"We spoke to the tour leader last night. The vehicle we left with has had three dings on the way and is being fixed and we have put them on a hire bus and they are currently on their way to Esfahan," he said.
"For some reason some are questioning the itinerary saying 'will we go here?' and 'will we go there?' But they have visited everything promised in the itinerary and are doing fine," he said.
Mr Creasey blames the bad publicity on the presence of a female journalist on board, and says that all 38 passengers who set out from London in September are still there. A second coach, also full, is just a few days behind OzBus One. The biggest drama there appears to be a love-struck Australian boyfriend who is flying out to intercept the bus after learning of some of the goings-on via the internet.
One blogger from the bus wrote last week: "17 days, 10 countries, 41 almost-strangers, countless litres of beer, hundreds of packs of cigarettes, much heavy petting, a few 'walks of shame'. A million rumours and a few true stories."
"The bus seat-rotation system now seems to apply to tents which is something I'm privy to most mornings as I am normally the first awake. Those who try to sneak back to their own tents/rooms when the sun rises are foolish to believe no one will see."
Last month The Independent reported from the bus as it began the epic journey. Danny Lawrence, 19, who has saved for two years to pay for the trip, said the atmosphere was jubilant after day one, which began at 7.24am on the Thames Embankment. "It's been a tiring day, but I still know it's the best decision I've ever made."
The OzBus website says of its service: "We are an adventure travel company like no other! We operate a regular overland service for backpackers travelling between London and Sydney.
"Whether you're heading off backpacking, or returning home from overseas, OzBus offers you the radical alternative to flying. This is a truly awesome experience and the ultimate overland adventure; become one of the few people ever to undertake this epic journey."
The company says that customers should be "like-minded" in spirit. "All that we require from you is that you have an adventurous spirit, a strong sense of fun and a longing to see the world as it really is," the website says.
The travellers – who are on the whole British, Australian and Irish – vary in age from around 19 to 69. Mr Creasey described them as: "natural travellers and probably more open to wacky ideas."
Four more buses are due to leave London next year with three scheduled to make the return journey from Down Under.