It seems some words get the little red squiggly even though they are valid words. If you add a space after them, the squiggly goes away. I have only seen this in custom templates; the default fields like Description do not exhibit this behavior.
Examples that erroneously squiggle:
Great
Just
Vest
Turtle
Pill
Dream
Dreamer
Hunter
Hunting
Island
Passing
Examples that do not erroneously squiggle:
Vests
Turtles
Pills
Dreams
Dreamers
Hunt
Hunts
Hunters
Isle
Islands
Pass
Passes
Everything I tried that ends in 's' does not squiggle. Beyond that, I don't see any pattern.