* os.texi (Timers): Avoid waiting inside timers.

lispref/ChangeLog

@@ -1,3 +1,7 @@
+2006-08-24  Chong Yidong  <cyd@stupidchicken.com>
+
+	* os.texi (Timers): Avoid waiting inside timers.
+
 2006-08-21  Lute Kamstra  <lute@gnu.org>
 
 	* Makefile.in: Use ../man/texinfo.tex to build elisp.dvi.

lispref/os.texi

@@ -1394,6 +1394,13 @@ both before and after changing the buffer, to separate the timer's
 changes from user commands' changes and prevent a single undo entry
 from growing to be quite large.
 
+  Timer functions should also avoid calling functions that cause Emacs
+to wait, such as @code{sit-for} (@pxref{Waiting}).  This can lead to
+unpredictable effects, since other timers (or even the same timer) can
+run while waiting.  If a timer function needs to perform an action
+after a certain time has elapsed, it can do this by scheduling a new
+timer.
+
   If a timer function calls functions that can change the match data,
 it should save and restore the match data.  @xref{Saving Match Data}.