It’s impossible to plan for every contingency– for example, the show probably won’t go on if there’s a major earthquake.  But there are some things you can anticipate.  Know what you’d do if you can’t show your slides for some reason, or how you could compensate if only a few people show up to a talk where you expected many.  Think about which parts of your presentation you might cut if you wind up with less time than you had planned for. Most importantly, have a plan for dealing with strong reactions if you’re discussing an emotional topic.  Sometimes continuing with your original plan isn’t appropriate and you just need to try something new.  Having a backup plan for these emotionally-charged events can be critical.