I noticed this on one of our F740's in the weekly e-mail:

Thu Mar 2 13:36:35 EST [adm1-na7: isp2100_main]: Loop break detected on ISP2100 in slot 3. Thu Mar 2 13:36:35 EST [adm1-na7: isp2100_main]: Loop back up on ISP2100 in slot 3.

There were no noticeable problems around that time, nor were any other messages reported. adm1-na7 is clustered with adm1-na8, both are running "NetApp Release 5.3.4: Fri Oct 15 12:28:26 PDT 1999". The FC-AL adapter in slot 3 is connected to adm1-na7's partner's drives though, which is probably why nothing bad happened. I'm more curious than concerned at this point, since this is the first time I've seen this happen (loop down/up event logged in the same second).