1) people who try to issue corrections (particularly spelling and grammar) are likely to commit the same offense themselves, usually in the actual post where they're making the correction.
2) Any correction of the speech or writing of others will contain at least one grammatical, spelling, or typographical error.
On a social network or forum:
"my mohter went out"
"dude, you mispelled mother"
"dude, you misspelled misspelled"
That's Skitts Law