Tree vs graph structure for task dependencies

[dbogdanov]

In my workflow, even though there is a single root node, a tree structure isn't sufficient: sometimes, you would have several tasks that can be addressed by a common subtask.

I wonder what your ideas on that are.

[xeruf]

Isn't that just a dependency? For me the tree is about grouping and common tasks, there may still be cross-dependencies.
Maybe an example would help.