Better correction :)

app/views/events/_event.haml

@@ -1,7 +1,7 @@
 %li.event{ title: display_date(event) }
   - if event.locality?
     %em.locality.fa.fa-globe
-  - elsif event.tags && event.tags.include? 'libre-en-fete'
+  - elsif event.tags && event.tags.include?('libre-en-fete')
     = image_tag 'lef-small.png', alt: 'LEF', width: 16, height: 16
   = link_to event do
     %strong.city{ title: event.address }= event.city

app/views/events/show.html.haml

@@ -1,6 +1,6 @@
 - set_event_meta
 
-- if @event.tags && @event.tags.include? 'libre-en-fete'
+- if @event.tags && @event.tags.include?('libre-en-fete')
   Cet événement s'inscrit dans le cadre de
   %a(href='http://www.libre-en-fete.net')
     = image_tag 'lef.png', width: '180', height: '60', alt: 'Libre en Fete',