The colloquial phrases 说到哪儿去了 and 想到哪儿去了
They have a similar meaning. They both are used in spoken language.
说到哪儿去了
The colloquial phrase 说到哪儿去了 
is used when someone gets away from the subject of a conversation.
is used when someone gets away from the subject of a conversation. Pay attention
In a sentence, this phrase can be preceded by a pronoun. In most cases, it is the pronoun 你 .
.Examples
说到哪儿去了，我说的买蛋糕的问题
You've strayed from the point. I was talking about buying cakes
shuō dào nǎr qù le, wǒ shuō de mǎi dàngāo de wèntí
看你说到哪儿去了，我们没有牵扯个人
That’s not the point. There's no need to get personal
kàn nǐ shuō dào nǎr qù le, wǒmen méiyǒu qiānchě gèrén
想到哪儿去了
The colloquial phrase 想到哪儿去了 is used when an opinion of one person differs from an opinion of another person.
is used when an opinion of one person differs from an opinion of another person.Examples
你想到哪儿去了！我肯定不会跟别人说的
How could you possibly think that? Of course I won't tell anyone!
nǐ xiǎng dào nǎr qù le! wǒ kěndìng bú huì gēn biérén shuō de
山本，你想到哪儿去了，作业当然会有的
How could you think that, Shanben? Of course I'll give you homework
shānběn, nǐ xiǎngdào nǎr qù le, zuòyè dāngrán huì yǒu de