Just saved me 30 mins work, phew!!

(This is where Outlook stores files you open from an email attachment, you know, that you then forget to save elsewhere after editing them...)

News to me! In Outlook check Tools>Options>Other>Advanced>COM Add-Ins. Apparently there for syncing with your calendar. No mention of this in the install. Naughty Apple! (this broke the SyncMyCal add-in for me)