Horizon

In my case it's the use of integers as constants when objects are persisted to a database. This is a holdover from old C enums that most developers have never seen any reason to change. The problem is that when you're able to toss around constants like DELETED, DO_NOT_REMOVE, or STATUS_UNKNOWN in your application, persisting them as meaningless ints means the poor schmos deploying and customizing the app end up staring at "7" and wondering whether tham means "do not remove", "already deleted" or "go throttle the developer who wrote this".

The irony is that the use of integers is an old performance enhancement thing that hasn't been useful for database performance for decades. The longest string constant will get converted into some some hashcode and dumped into a bucket where it can be consulted as quickly as an integer would. The only difference is whether you want the deployment folks to hate you or not.

A more commonly cited set of confused concepts are inheritance, encapsulation, and when to use each.

java.util.Properties is a notorious example of the use of inheritance when encapsulation is far more appropriate.

That OO stuff is fascinating. I wish I had enough time these days to sit down with Design Patterns on a rainy Sunday and geek out.