This shows the improper clip placement of an angled permanent clip on a large posterior communicating artery aneurysm (Pcomm). The clip is not long enough to cover the aneurysm neck so as blood travels through the internal carotid artery, the small opening in the neck creates a "hose effect." This effect is similar to partially covering a running hose with your thumb. The water will come out at a very high pressure. Just like the hose, blood moving into the aneurysm is doing so at very high pressure which causes the aneurysm to burst.
Anatomy in this medical illustration: Anterior cerebral artery section one (A1), Middle cerebral artery section two (M1), Posterior communicating artery, Internal carotid artery, Aneurysm