I was in this case:

<component :is='componentName' :foo='foo' ></component>

And the component pointed had invalid template, leading to this error:

[Vue warn]: Attribute ":foo" is ignored on component <component> because the component is a fragment instance: http://vuejs.org/guide/components.html#Fragment-Instance

Now this error also gives the name of the component that failed:

[Vue warn]: Attribute ":foo" is ignored on component <component:myComponent> because the component is a fragment instance: http://vuejs.org/guide/components.html#Fragment-Instance