Anti-pattern: Overuse and misuse of macros. (Or perhaps not a "pattern", since the manifestations are so varied?)

For non-Elixir folks, imagine something that looks like a regular function-call where expressions are evaluated and the results are passed in... but in reality all arguments are symbolic inputs to something that emits new code.

For example, you might see this unassuming line in your editor:

    describe_combo(a(foo),b(bar))

But what might actually be compiled is:

    # What might actually get compiled
    # Runtime results like "a(1)+b(2)=3"
    "a(" <> to_string(foo) <> ")+b(" <> to_string(bar) <> ")=" to_string(a(foo)+b(bar))

And if someone in your codebase is an evil misanthrope:

    foo = "haha I changed your local variable"
    IO.puts("surprise, suckers") # Prints to console
    kick_puppies()

Macros are actually part of the core DNA of the language and underpin how some of its syntax works... but with great power comes great potential for WTF.