schoolboy errornoun [ C ]
uk/ˈskuːl.bɔɪ ˌer.ər//ˌˈskuːl.bɔɪ ˌer.ɚ/UK US rookie mistake, /ˌrʊk.i mɪˈsteɪk//ˌrʊk.i mɪˈsteɪk/a basic mistake, like one that a person with no experience of an activity would make:
Sending the wrong message to the wrong group is a schoolboy error you will avoid with the help of our app.