by mistake是因为失误的意思，而by accident则是因为意外的意思，区别可以从以下例子看出：I put you in the wrong place by mistake, sorry for that. As there are many applications, I mixed your name with others, put you in any group of people by accident. 我错误地把你放在了错误的地方，对不起。

由于应用程序很多，我把你的名字和其他人混在一起，不小心把你放在任何一群人中。