An attribute ID is not allowed for subgraph. But graph in subgraph can have it's own ID.

For example this file has syntax error in it:
digraph "unnamed" {
graph [fontsize="11",id="unnamed"]
subgraph cluster_X1  {id="cluster_X1",label="X1",cluster_X1_T1_1
[comment="T1",fontsize="10",id="cluster_X1_T1_1",label="T1",name="T1"];
}
}

While this one is correct:
digraph "unnamed" {
graph [fontsize="11",id="unnamed"]
subgraph cluster_X1  {
graph[id="cluster_X1",label="X1"]
cluster_X1_T1_1
[comment="T1",fontsize="10",id="cluster_X1_T1_1",label="T1",name="T1"];
}
}

REVIEW: 102968

Many thanks to Ivan Brezina for this patch