Nerves. For a lawyer they might crop up during a particularly tricky case, for others a time when you have to speak in public, or even just go to the dentist!
For Sammi Kinghorn, things tend to come to a head when she takes her place on the track. Mind you, that's understandable when you're a 20-year-old under the global, live-TV spotlight of your first ever Paralympic performance.
Sammi managed a kiss and a wave to the camera as it panned along the competitors on the start line, but inside she was simply trying to stay focussed.