A somewhat rare variant is the median artery of the corpus callosum that originates directly from the anterior communicating artery. It is present in approximately 2-3% of the population. Its presence is associated with an increased risk of cerebral aneurysms.

Here is an example of a median artery of the corpus callosum with a fenestration at its origin.

Here is a dissected cadaver specimen (via this article) indicating a rare duplicated median artery of the corpus callosum: