There's a million things that can go wrong. Maybe "wrong" is the wrong word. There's a million ways to be inconsistent. Even among really good brewers, I don't think they brew every day or even every week for the most part. On top of that they hardly ever brew the same beer twice in a row. So it's tough to be consistent. It can be done, but it's very difficult to do.
Sure, re-brewing recipes and brewing frequently can help develop consistency, but understanding how to control the variables and planning are just as important as practice. I may not be coming from the same place as a lot of brewers, since I have developed a lot of procedures to make biological processes reproducible, but I don't find brewing to be that complex or difficult to control.
Like most brewers here, though, I assume, I start with a target mash temperature, volume and OG and hit them within a few percent. Of course I'm not constantly changing my equipment, I usually brew with a limited set of base malts that I like, and I almost always work within a 2-fold range of OGs. I'm not a "seat of the pants" brewers, either, and wouldn't really have any interest in brewing without having planned everything out to make sure that things don't go awry.