Update docs

docs/tut/crash-course/12-strict-mode.md

@@ -5,5 +5,27 @@ be set with `vide.strict = true` once when you first require Vide. Strict mode
 will add extra safety checks and emit better error traces, particularly when
 errors occur in property bindings.
 
+Strict mode will run derived sources and effects twice each time they update.
+This is to help identify improper cleanup of side-effects and ensure that pure
+computations are actually pure.
+
+```lua
+local source = vide.source
+local effect = vide.effect
+
+vide.strict = true
+
+local count = source(0)
+
+local ran = 0
+effect(function()
+    ran += 1
+end)
+
+print(ran) -- 2
+count(1)
+print(ran) -- 4
+```
+
 A full list of what strict mode will do can be found
 [here](../../api/strict-mode).