Always be thinking ahead, and having a contingency plan, or a way out. No matter how rudimentary the flight, or how many times you've flown the route before.
For example - i'm flying on a perfectly clear summers day - late in the afternoon, CAVOK - as good as flying weather gets. Last leg back home, sitting nicely in the cruise at 8,500 feet.
As I proceed along the planned route, "cloud" appears on the horizon. Topping out somewhere either at my level or above it - and going all the way to the ground. A wall of cloud....How could this be I thought ? on such a perfect summers day?
After further investigation, and getting closer to it, I see it is in fact smoke, from a huge bushfire up north.
Winds aloft are sending the wall of smoke South for well over 200nm. Its at least 70nm or more North to get around where the fire has started. Realisation sets in, as do the questions...do I have enough fuel to go around it? Can I go over it? I cant go through it..... Will I have enough fuel to divert somewhere else? Will I have enough daylight?
In this particular instance, it did not end up an emergency, but it was going to be a pain in the ass diverting elsewhere when you need to get back somewhere - or going elsewhere to get more fuel so you can make it around the smoke.
I was so convinced before that leg that in perfectly clear CAVOK weather and not in an area or season known for bushfire, that something like that could ever happen.
A fairly tame issue really, but it did happen to me, and was an excellent wake up call for when you inevetiably end up in your "comfort zone" after flying for many hours with everything running smoothly.
In different circumstances, and with less fuel on board, it could have gotten alot more serious, real quick.
It acted as a stern reminder to me that no matter how good the conditions on the forecast, or outside the window look, there is more to flight & fuel planning than simply assuming the only fuel you need in good weather is from A to B.