I always get definitely, and defiantly mixed up.... >.> (weird ik)
it's because I always spell definitely wrong to begin with, so when I spell-correct it, it gives me defiantly, instead.
There, their, and they're. I usually don't proofread until I submit. Even though I tell myself: their is for people, there is for places, and they're is they are but I still derp at times.