Reviewed another paper. Had the unfortunate experience of believing the conference tool when it told me that "a ConfTool session is limited to two hours"; after 45 minutes of work, I submitted my review, only to be bounced out to the login screen with the loss of my data. Reported this to the admin email on the site, and then reconstructed my review from memory.

In future I'll write the review in a text editor and save it before trying to submit it.