diff -r 02e7e1d73ebb -r 75f44d3063da tests/quadrilaterals.py
--- a/tests/quadrilaterals.py	Wed Jan 24 23:14:01 2018 +0200
+++ b/tests/quadrilaterals.py	Wed Jan 31 14:34:23 2018 +0200
@@ -1,5 +1,5 @@
 from math import radians
-from testsuite import warning, error
+from testsuite import warning, error, notice
 from geometry import *
 
 def sign_consistency(container):
@@ -58,12 +58,26 @@
                 if cross_product(b - a, c - a).length() < 1e-5:
                     yield error(element, 'collinearity-error')
 
+def hairline_score(smallest_angle):
+    from math import log10
+    return max(0, -log10(smallest_angle))
+
+def hairline_test(model):
+    for element in model.body:
+        if hasattr(element, 'geometry') and len(element.geometry.vertices) >= 3:
+            smallest_angle = element.geometry.smallest_angle
+            if smallest_angle < radians(0.5):
+                yield notice(element, 'hairline-warning',
+                    smallest_angle = smallest_angle,
+                )
+
 manifest = {
     'tests': {
         'skew': skew_test,
         'concave': concave_test,
         'bowtie': bowtie_test,
         'collinearity': collinear_test,
+        'hairline': hairline_test,
     },
     'messages': {
         'skew-error': lambda skew_angle:
@@ -77,5 +91,9 @@
         'concave-error': 'concave quadrilateral',
         'self-intersecting': 'bowtie quadrilateral',
         'collinearity-error': 'collinear polygon',
+        'hairline-warning': lambda smallest_angle:
+            str.format('hairline polygon (smallest angle {})',
+                degree_rep(smallest_angle),
+            ),
     },
 }