You could fall. You could eat something fattening. You could trip over somebody's laptop bag and bang your head on a sharpened napkin-holder. You could spill your coffee on your shirt.

Oh yeah... and if you don't keep your machine up-to-date with patches, or send private information over an insecure (http) connection, or if you're extremely unlucky and happen to be on the same wifi node with someone who is being malicious, or some malicious infected machine that is capable of exploiting a hole in your machine, something bad could happen.

I wouldn't worry too much if the patches are up-to-date and you're using common sense regarding what web sites you visit.