To Reproduce:
1. Create a script with this code:

using UnityEngine;
using System.Collections.Generic;
public class Test : MonoBehaviour {
public List<NumbersInSpanish> theList;
void Start () {
theList = new List<NumbersInSpanish>();
theList.Add (new NumbersInSpanish(){value1 = 1, value2 = "Uno"});
theList.Add (new NumbersInSpanish(){value1 = 2, value2 = "Dos"}); // Break here
}
}
public class NumbersInSpanish {
public int value1;
public string value2;
}

2. Attach the script to an GameObject. Attach MonoDevelop to the Editor and place a breakpoint in the line with the comment.
3. Enter play mode and try to inspect the "theList" variable. MonoDevelop with display the following error:

Unknown type 'System.Collections.Generic.CollectionDebuggerView`1, mscorlib.dll'