It all depends on what one means by "already been shown how to do it". Something like that could easily be misunderstood/forgotten by the trainee or glossed-ove

It all depends on what one means by "already been shown how to do it". Something like that could easily be misunderstood/forgotten by the trainee or glossed-over by the senior. Or maybe he was just having a bad day. Whatever the case, learning often takes some amount of repetition before mastery is achieved. Real life work is not like stackoverflow, if you have a team that's your responsibility, you can't just downvote them to oblivion. Nor can you use performance on any one incident to decide to end someone's career in that particular workplace. Sometimes people just don't measure up to expectations, that's OK, it happens, but firing should be a decision based on a history of underperformance. Questioning whether or not someone is fit to "be there" should be something that is done upon deciding to start a PIP. Otherwise, it's just eroding the confidence and motivation of the individual and the team and, in many ways, is as bad as firing that person out of spite.