Thank you to my mother, for catching what I hope is the very last typo in INVICTUS: CRISIS.
I read over this word, purposefully, many times without thinking anything of it. No other early reader flagged it. My mother didn’t notice it the first time she proofread this book. But:
Purposefully: in a way that shows determination or resolve.
Purposely: on purpose; intentionally.
It’s not like I didn’t know that! Of course I knew that! But I checked synonyms for both words anyway to confirm that I needed to switch words.
There’s some other word pair just like this … what is it? … oh, publically vs publicly, except here one is a real word (but looks wrong) and the other is a rare variant (that looks right to me). Do you know which is which at a glance? I don’t. Someone … Linda S? … anyway, some excellent proofreader caught this one for me a couple of times and now I know which is right, but I still have to pause before I type it because it does not look right.
That’s from a blog called Grammarphobia.
[S]ome standard dictionaries do include “publically” as a variant spelling, but it’s described as less popular than “publicly.” In fact, “publicly” outnumbers “publically” by more than 100 to 1 in Google searches.
A hundred to one! Why is “publicly” preferred? The linked post does not explain this, though it does go into some detail regarding adverbs of this type. It’s a fine post, but still, I do wonder why this one -ic adjective is different from all the other -ic adjectives, such as “comic” and “rustic.”
I do think the difference between “purposefully” and “purposely” is more clear, but I’m not sure dictionaries do a good job explaining the difference. Here’s a good explanation from the Grammarist webiste:
Purposely means on purpose. It is a synonym of intentionally. When you mean to do something, you do it purposely.
Purposefully means (1) with a sense of purpose, or (2) with determination. For example, when you are determined to ask your boss for a raise, you might walk into her office purposefully.
Bold is mine because I think those phrases make the difference clear.
Anyway! I hope switching purposefully to purposely is the very last correction I’ll be making to Invictus: Crisis!

Well, most things one does with determination are also done on purpose.
So I can understand a proofreader not querying whether you really meant the action was done with a sense of determination, and not just consciously chosen…
I am not sure if I explicitly know the word “purposely”, though I certainly would know what it meant if I saw it. I would invariably use “on purpose” or a synonym like intentionally.