Today I learnt that you can easily make a network loop on a Dell server with an embedded switch, causing a network outage for the whole office.
The embedded switch has a few physical external ports and a lot of logical internal backplane ports. If you use port mirroring on the external port but you send the mirrored frame also on the external port, the copied (mirrored) frames will multiply like crazy, because the frame that wants to go out does go out but is first copied. Its copy also wants to go out is is first copied etc. etc. One frame quickly becomes a billion frames that go out to your LAN.
Another example of how you can cause a loop by using SPAN is by ”faking” RSPAN: