Docs say that the default ID on tell-a-friend forms is “tellafriend_form”
http://ellislab.com/expressionengine/user-guide/modules/email/tellafriend.html#form-id
On an upgrade from an EE1 installation I’m seeing this tag defaulting to an ID of “contact_form”. I imagine that I can fix it by specifying the ID in the tag, so just reporting.