• Writing the genitive form of it as “it’s”. It’s also common to see it the other way around, writing the contraction of “it is” as “its”
• Finding the right use for there, their, and they’re
• On a more general basis, the past sense of to be seems to be written as were even though it’s rarely plural or first person singular
• You’re vs. your. Actually, this seems to be rather general for these contractions
• Adding a genitive ‘s to words that end in -s.

I might write more of these later on, when I stumble upon them.