This is a usability problem I encountered today:
I installed the new Version of “Low Variables” which comes with a new fieldtype. In Add-Ons > Fieldtypes I Clicked “Install” next to the new Fieldtype. In the next screen (see attachment) “Install” was already preselected. I ignored the other two settings, because in my head I was concerned with the Fieldtype and not the Extension of Module. I clicked “Submit”.
BOOM, Low Variables gone, all variables deleted. No warning.
Luckily this was on my local test-installation. There should really be another warning, or the radio buttons should all be on “Install”.