To reproduce:
1. Create a new script
2. Add these lines to the script:

public Object[] testus;

void Start() {
ArrayList traList = new ArrayList ();
traList.CopyTo(testus);
}

3. Build & run for WP8
4. The build fails with:
Error building Player: Exception: Error: method `System.Void System.Collections.ArrayList::CopyTo(System.Array)` doesn't exist in target framework. It is referenced from Assembly-CSharp.dll at System.Void NewBehaviourScript1::Start().