"By accident/by mistake" in Japanese
What's the best way to say "by accident" in Japanese?

Eg.
I sat in his chair by accident.
I sat in his chair by mistake.
or
She accidentally called me Phil.

Something like:
偶然で彼の椅子に座ってしまった。
彼の椅子に誤って座った。

What do you think?

Question

"By accident/by mistake" in Japanese
What's the best way to say "by accident" in Japanese?

Eg. 
I sat in his chair by accident.
I sat in his chair by mistake.
or
She accidentally called me Phil.

Something like: 
偶然で彼の椅子に座ってしまった。
彼の椅子に誤って座った。

What do you think?

Answer

Hi Matt
Your translation is very good! I think you can say both meanings  for "by accident" as you said.
 So,

偶然に彼の椅子に座った。or
誤って彼の椅子に座った。
We need to find the meaning by seeing the flow of conversation.

As to "by mistake",
There is no implication of "coincidence". So we can say simply,

誤って彼の椅子に座った。

I hope my answer helps you.