When checking in a versionable node, that node and its subgraph should be read-only. In particular, Section 15.2.2 of the JCR 2.0 specification has the relevant bits:

When a versionable node is checked in, it and its subgraph become read-only. The effect of read-only status on a node depends on the on-parent-version (OPV) status of each of its child items.

When a node N becomes read-only:

• No property of N can be added, removed or have its value changed unless it has an on-parent-version setting of IGNORE.
• No child node of N can be added or removed unless it has an on-parent-version setting of IGNORE.
• Every existing child node of N becomes read-only unless it has an on- parent-version setting of IGNORE or has an on-parent-version setting of VERSION and is itself versionable.

Apparently we restrict the adding/modification/removal of properties (tho we should verify that with additional tests), but we do not restrict the adding or removal of nodes. Additionally, even when the OPV setting of a child node definition is VERSION, the expected behavior differs depending upon whether the actual child node is versionable.

Consider these node types:

[A] > mix:versionable
  - name (string) copy
  + b (B) multiple version

[B]
  - name (string) copy

and this code:

Node root = session.getRootNode();
Node a    = root.addNode("node", "A");
a.setProperty("name", "aaa");
session.save();
versionManager.checkin(a.getPath());
// The next call is correctly prevented ...
// a.setProperty("name", "aaa");

// But the next call should result in an exception, but doesn't
Node b = a.addNode("b", "B");
session.save();

// This shouldn't be possible either ...
b.remove();

session.save();

The a.setProperty("name", "aaa") statement (commented out) is not possible without an exception:

javax.jcr.version.VersionException: '/node' (or its nearest versionable ancestor) is checked in, preventing this action
          at org.modeshape.jcr.JcrNode.doRemove(JcrNode.java:118)
          at org.modeshape.jcr.AbstractJcrNode.removeShare(AbstractJcrNode.java:2415)
          at org.modeshape.jcr.AbstractJcrNode.remove(AbstractJcrNode.java:2458)
          at au.com.infomedix.documentstore.cnd.TestLoadCND.main(TestLoadCND.java:88)

However, the a.addNode("b", "B") should not be possible either without an exception. And once there is a child named 'b', whether its properties can be changed depends on whether 'b' is versionable. Currently, setting a property on 'b' when 'a' is checked out is not currently possible, even when 'b' is versionable.

See the forum post for more context and detail.