Setting up a new switch in the office, and to show the pretty light-show on startup, rebooted the switch. Turns out that the newer Procurve 1810Gs don't automatically save the config (you have to tell them to after making changes), and they don't have loop protection on by default...
So reboot -> no loop protection and that 3-cable LACP trunk I'd just put them on was suddenly a bit of a loop[1]...
Moral of the story : Don't assume that (what seems like) a minor version update in a product doesn't change the behaviour substantially (the 1800G switches save the config when you make changes, the 1810Gs have it as an explicit step).
[1] A gigabit loop on a completely flat network, so there was essentially a gigabit of broadcast traffic pounding at the ~100 machines on the network...
So reboot -> no loop protection and that 3-cable LACP trunk I'd just put them on was suddenly a bit of a loop[1]...
Moral of the story : Don't assume that (what seems like) a minor version update in a product doesn't change the behaviour substantially (the 1800G switches save the config when you make changes, the 1810Gs have it as an explicit step).
[1] A gigabit loop on a completely flat network, so there was essentially a gigabit of broadcast traffic pounding at the ~100 machines on the network...