ことはない

ことはない after a verb in the dictionary form means "there's no need to": you tell someone they don't have to do something, usually to reassure them. The polite form is ことはありません.

Said about something already done or being done, it turns into a mild reproach: you didn't have to go that far.

ことはない or なくてもいい

なくてもいい simply gives permission not to do something. ことはない says the action is pointless or more than the situation calls for, so it suits comfort and gentle criticism, and sounds odd as a plain statement of what's optional.

"Never happens"

About a third person or an event, ことはない can also say that something does not happen at all. Context tells the two meanings apart.

ことはない or たことがない

With a た-form and が, the pattern is たことがない, "have never done": an experience. ことはない takes the dictionary form and is about need, or about what never happens.