I dyed my hair jet black recently but missed huge chunks in the back of my head which I didn't realize until people started pointing it out to me in the days and weeks afterwards. It's not like I was going from blonde to black.. I was going from dark brown to black so it wasn't a huge difference.
But my biggest hair sin ever was years ago when I used to bleach my hair and dye it with special effects dye. After I had been dying my hair purple and ronald mcdonald red for months, I decided to let it grow out. My purple hair had faded to pink and then the pink faded to some shitty hideous peach orange, and for some insane reason I just kinda kept it that color for months and let it grow out. I have no idea what I was thinking.