diff -u b/core/includes/common.inc b/core/includes/common.inc
--- b/core/includes/common.inc
+++ b/core/includes/common.inc
@@ -1199,11 +1199,11 @@
  * into script execution a call such as set_time_limit(20) is made, the
  * script will run for a total of 45 seconds before timing out.
  *
- * It also means that it is possible to decrease the total time limit if
- * the sum of the new time limit and the current time spent running the
- * script is inferior to the original time limit. It is inherent to the way
- * set_time_limit() works, it should rather be called with an appropriate
- * value every time you need to allocate a certain amount of time
+ * If the current time limit is not unlimited it is possible to decrease the
+ * total time limit if the sum of the new time limit and the current time spent
+ * running the script is inferior to the original time limit. It is inherent to
+ * the way set_time_limit() works, it should rather be called with an
+ * appropriate value every time you need to allocate a certain amount of time
  * to execute a task than only once at the beginning of the script.
  *
  * Before calling set_time_limit(), we check if this function is available