But it was impossible to paint the scene on location for a simple reason: The only place to set my stool was a busy sidewalk that was only two feet wide. 
Because the road was so narrow, and because it was the main thoroughfare for the coastal route out of Bantry, huge trucks were forced ride up onto the sidewalks to allow other vehicles to pass. There was a real risk of being clipped by a rear-view mirror, and I didn’t want to force pedestrians to step into the street. So I had to give it up and look for another motif.
I call such an impediment a “gamestopper.” It’s something that shuts down a plein-air effort, and it has nothing to do with issues of technique or composition.
I’ll list some other examples, and I’m sure you’ll have more in the comments. Every one of these has happened to me:
1. Menaced by bull in the middle of a field.
2. Heavy downpour starts (fatal to watercolor) and wind blows rain under umbrella (eventually shuts down oil painting).
3. Forgot the chair, and no place to sit down.
4. High wind makes easel impossible to set up.
5. Subject (person, vehicle, animal) departs.
6. Drunk guy in bar keeps bumping hard into sketching arm.
7. Tide comes in, eliminating setup area.
8. Donkey puts head in lap.
9. Sketching from drawbridge; drawbridge lifts.
10. Goats keeps nibbling sketchbook.
11. Kicked out by guard/ harbormaster/ cop/ farmer/ railyard bull—and once ejected by a nun!
12. Easel blown over washed down waterfall.
13. Scheduled steam train (my ride home) must depart.
14. Folding chair collapses in museum.
15. Biting insects become too unbearable.
16. Unseen people on overlook above keep spitting on me.
17. Forgot key supplies (brushes, solvent, paints, or panel).
18. Fog comes in and covers view.
19. Shop opens or doorway becomes active.
20. Sub-freezing temperatures freeze watercolor.
21. Automatic sprinklers turned on in garden.
22. Car or truck parks in front, obstructing view.
23. Lights turned on, killing mood; or turned off, obscuring sketchbook.
24. Tour bus unloads gaggle of annoying tourists who hover around snapping pictures and asking inane questions.
25. Portrait subject approached, waving a finger, superstitious about being drawn.
