accidentally vs by mistake
When I was looking conversation between my friend and I on chatbox I sent stamp (mark) accidentally.
I wanted to tell my friend it was mistake,no meaning
Can I say" I set it accidentally" or I should say by mistake??

And What is different between by accident and by mistake?Thank you everyone!

Question

accidentally vs by mistake
When I was looking conversation between my friend and I on  chatbox I sent stamp (mark) accidentally.
I wanted to tell my friend it was mistake,no meaning 
Can I say" I set it accidentally" or I should say by mistake??

And What is different between by accident and by mistake?Thank you everyone!

Answer

You can say it either way. An accident is something that you don't expect to happen, and don't mean to do.  A mistake is an error.  So hitting the wrong button was a mistake, as well as an accident.

Answer

You can use either phrase.

Answer

As far as I can tell, they're the same thing.

A very subtle difference is that an 'accident' is unintentional, but a 'mistake' is not necessarily (it could also have been a poor, but intentional, decision).
Here, 'by mistake' clearly means 'accidentally'.
It's a bit more natural to say "It was an accident" in this case, in my opinion.