Due to a seat I couldn't get adjusted, another seat that kept adjusting itself and a very very busy last bit of my shift, the area between my shoulders had started hurting. Which then travelled up to my head and stayed there.
However, I really knew it was a bad day when I couldn't get the company car in gear. I tried and tried and was cursing and grumbling. I turned the engine off and on again and still nothing.
Until I realised I had to actually use the clutch as well. That's when I really knew.
PS: it rained all day as well...