Simple Past versus Present Perfect

I don't fully understand whether to use present perfect tense or imperfect (simple past tense).

What's the difference between:

An error has occured. (present perfect)

and

An error occured. (imperfect)

I cannot tell any difference in meaning. Why choose one tense over the other, please?

It's not my preferred dictionary, but Merriam-Webster's 11th Collegiate Dictionary defines imperfect as "of, relating to, or constituting a verb tense used to designate a continuing state or an incomplete action especially in the past".

The same source defines present perfect as: "of, relating to, or constituting a verb tense that...expresses action or state completed at the time of speaking".

I'm a bit unsettled with the latter definition.

Thank you