This is something that when it happens can only crop up in a bad dream later on. As with anything, what ever it is that may produce some kind of fear, stays with us in our subconcious. The fear associated with being left behind is faily intense if you suddenly find yourself alone in a strange place without transportation or the tour you were accociated with.
As with anything in life, we have to be prepared for the unexpected, and in the case of missing your tour bus, preplanning for such a problem would greatly diminish the consequences associated with being late for the tour. Well as a precaution for such events, a little of that "pre-planning" would go a long way in solving any situation that may crop up.
First and foremost, a fully charged cell phone, and a spare battery is a must. You should have all the numbers of those in charge of the tour, as well as those who you know who left you behind. Not only should you have the numbers from the tour guide, but also for someone at the guide headquarters. Even the numbers of other tourists that you've made an acquaintance would help.
A vacation, especially with a tour guide, can be a really wonderful experience. However, being left behind can pretty much ruin the day, and possibly the whole vacation. I don't think anyone plans on being late for a tour, but it does happen, and it usually is a result of poor judgement of time. All can be prevented if everything is accounted for when it comes to schedule, preparedness, and a willing to take the extra steps necessary before hand for such a situation.
It is a very lonely sinking feeling when this happens, and the last place you would want it to happen is in a very strange, unfamiliar place where there are not a lot of conveininces. Depending on the weather, just in case, if your going to be stranded, then you might consider something that will protect you against the elements, be it cold, rain, or heat.
Getting back to being prepared, fortified with the numbers, it's a matter of calling the tour guide, and if not too inconvenient, they can swing back for you.
If not, then you still have numbers to call whereas you may find a way to meet up with them. So as long as it's thought that anything can happen, just plan on it, and maybe it will. But at least you were prepared. Did I mention the umbrella? It's starting to rain.