I mean it was careless of me to make such a wrong guess.

我是说我真不小心，竟然这样看错了人。

It was thoughtless of me, it was careless of me, an example?

是我大意了，是我大意了，举个例子？

It was careless of him to leave the door unlocked.

【careless，carelessly】他忘了锁门，真是粗心大意。