Hi Im using Arduino_18, just now i tried to use Watchdog timer_30MS and guess what, it makes 13 blinking all the time. Now, i cant upload anything to my arduino mega.
Some one, plz save me.... 
Yes this is because the watchdog kicks in before there has been time to recognise the download.
The only way back is to re burn your bootloader (and fuses) with a programmer.
What and how programmer should i do?
Playground ArduinoISP
See:-
Thx 
Sounds like my issue here...
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1258251558/6#6
There may be an alternate to reburning bootloader.