I can imagine someone using component entity systems where they don't exactly make sense. I worked with a manager who considered using it, under a different name and not tied to game dev, in a very LOB application because he thought the requirements were changing too rapidly to manage in a traditional way.

Another possible artifact would be a custom memory allocator or patterns around memory or object management that make no sense in an application that isn't memory constrained or allocating large amounts of objects in such a way that is causing a bottleneck.

Maybe the programmer was unrolling loops in the name of maximizing performance where it wasn't actually required.

I am sure there are plenty of other signs a game dev hopeful would leave in a code base if left unchecked.