diff --git a/terraform/context_plan_test.go b/terraform/context_plan_test.go
index 8778e4221..e58dbdf9f 100644
--- a/terraform/context_plan_test.go
+++ b/terraform/context_plan_test.go
@@ -1088,6 +1088,10 @@ func TestContext2Plan_dataResourceBecomesComputed(t *testing.T) {
 		t.Fatalf("missing diff for data.aws_data_resource.foo")
 	}
 
+	// This is added by the diff but we want to verify that we got
+	// the same diff as above minus the dynamic stuff.
+	delete(iDiff.Attributes, "id")
+
 	if same, _ := p.ReadDataDiffReturn.Same(iDiff); !same {
 		t.Fatalf(
 			"incorrect diff for data.data_resource.foo\ngot:  %#v\nwant: %#v",
diff --git a/terraform/resource_provider.go b/terraform/resource_provider.go
index 37cd1d5c3..542f14a61 100644
--- a/terraform/resource_provider.go
+++ b/terraform/resource_provider.go
@@ -3,6 +3,12 @@ package terraform
 // ResourceProvider is an interface that must be implemented by any
 // resource provider: the thing that creates and manages the resources in
 // a Terraform configuration.
+//
+// Important implementation note: All returned pointers, such as
+// *ResourceConfig, *InstanceState, *InstanceDiff, etc. must not point to
+// shared data. Terraform is highly parallel and assumes that this data is safe
+// to read/write in parallel so it must be unique references. Note that it is
+// safe to return arguments as results, however.
 type ResourceProvider interface {
 	/*********************************************************************
 	* Functions related to the provider
diff --git a/terraform/resource_provider_mock.go b/terraform/resource_provider_mock.go
index 8389fd0ae..f8acfafa9 100644
--- a/terraform/resource_provider_mock.go
+++ b/terraform/resource_provider_mock.go
@@ -157,7 +157,7 @@ func (p *MockResourceProvider) Apply(
 		return p.ApplyFn(info, state, diff)
 	}
 
-	return p.ApplyReturn, p.ApplyReturnError
+	return p.ApplyReturn.DeepCopy(), p.ApplyReturnError
 }
 
 func (p *MockResourceProvider) Diff(
@@ -175,7 +175,7 @@ func (p *MockResourceProvider) Diff(
 		return p.DiffFn(info, state, desired)
 	}
 
-	return p.DiffReturn, p.DiffReturnError
+	return p.DiffReturn.DeepCopy(), p.DiffReturnError
 }
 
 func (p *MockResourceProvider) Refresh(
@@ -192,7 +192,7 @@ func (p *MockResourceProvider) Refresh(
 		return p.RefreshFn(info, s)
 	}
 
-	return p.RefreshReturn, p.RefreshReturnError
+	return p.RefreshReturn.DeepCopy(), p.RefreshReturnError
 }
 
 func (p *MockResourceProvider) Resources() []ResourceType {
@@ -214,7 +214,15 @@ func (p *MockResourceProvider) ImportState(info *InstanceInfo, id string) ([]*In
 		return p.ImportStateFn(info, id)
 	}
 
-	return p.ImportStateReturn, p.ImportStateReturnError
+	var result []*InstanceState
+	if p.ImportStateReturn != nil {
+		result = make([]*InstanceState, len(p.ImportStateReturn))
+		for i, v := range p.ImportStateReturn {
+			result[i] = v.DeepCopy()
+		}
+	}
+
+	return result, p.ImportStateReturnError
 }
 
 func (p *MockResourceProvider) ValidateDataSource(t string, c *ResourceConfig) ([]string, []error) {
@@ -245,7 +253,7 @@ func (p *MockResourceProvider) ReadDataDiff(
 		return p.ReadDataDiffFn(info, desired)
 	}
 
-	return p.ReadDataDiffReturn, p.ReadDataDiffReturnError
+	return p.ReadDataDiffReturn.DeepCopy(), p.ReadDataDiffReturnError
 }
 
 func (p *MockResourceProvider) ReadDataApply(
@@ -262,7 +270,7 @@ func (p *MockResourceProvider) ReadDataApply(
 		return p.ReadDataApplyFn(info, d)
 	}
 
-	return p.ReadDataApplyReturn, p.ReadDataApplyReturnError
+	return p.ReadDataApplyReturn.DeepCopy(), p.ReadDataApplyReturnError
 }
 
 func (p *MockResourceProvider) DataSources() []DataSource {