After installation of Jetpack, the following line appeared on my website homepage only (theme: Academica):
Warning: Invalid argument supplied for foreach() in /home/mellov/ideasinpolitics.com/wp-content/themes/academica/header-image.php on line 15
Uninstalling Jetpack solved the problem. When I reinstalled the plugin the warning line reappeared but this time on a blog post and not on my homepage or static 'about the author' page.
I am hoping that there is some way to keep Jetpack installed as other than this it seems to be a very promising plugin.