Sorry to hear on your genuine mistake.  

Just take this as a once in life time lesson.

then see that you are not the only one, read the SGX today's buy in report, you can see many others doing similar mistake. see some of them also one lot naked short , for example Federal, multichem, tritech

Buying-in Executed on December 15, 2009
SECURITY TOTAL QTY EXECUTED CURR TOTAL VALUE
BAN JOO 100,000 S$ 8,000.00
BENG KUANG 2,000 S$ 620.00
CAPITALAND 10,000 S$ 42,700.00
CAPMALLSASIA 2,000 S$ 5,440.00
CHARTEREDSC 500 S$ 1,350.00
CHINA ENERGY 40,000 S$ 10,200.00
CHINA XLX 5,000 S$ 3,525.00
EASTGATE 400,000 S$ 8,000.00
FEDERAL 1,000 S$ 210.00
GLOBAL YELLOWPGS 26,000 S$ 4,290.00
GOODPACK 3,000 S$ 3,450.00
HO BEE 8,000 S$ 12,720.00
KEPT&T 5,000 S$ 7,000.00
LI HENG 100,000 S$ 28,000.00
MATEX 25,000 S$ 2,250.00
MULTICHEM 1,000 S$ 220.00
OLAM 5,000 S$ 13,300.00
PAN HONG W121027 5,000 S$ 850.00
SING TEL 990 S$ 3,032.37
STI2850SGP100128 100,000 S$ 24,500.00
TRITECH 1,000 S$ 305.00
UTDENVTECH 50,000 S$ 16,500.00
WING TAI LTD 7,000 S$ 12,670.00
======================================
TOTAL S$ 209,132.37

I really worry for newbies learning to short.  The rules of the Stock Exchange do not allow naked (uncovered) shorting of shares.  It allows shorting that is covered (backed up by deliverable shares) otherwise the shortist will have to pay a penalty on due date of delivery which is 2 and a half day.  After two and a half day, if one cannot deliver (meaning one has no credit balance in his CDP account) a buy-back will be instituted and a penalty of $1.5K plus other misc fees imposed.

To "beat" the system, a trader can do two things which will not create a penalty or a buy-back.  The first is to short (sell) a share and thenbuy back (win or lose) on the same day before the market closes.  Secondly, the trader can open a SBA (share borrowing account) or also known as SBL (Share borrowing and Lending) account.  The term will differ with brokerages.  Just have to ask them how to open the share borrowing account. After you have the SBA/SBL acct, you must know what shares you want to borrow to short. Borrow the shares first will be safer because some shares are not loanable.  Remember without a SBL or SBA, you cannot short sell, because you still do not have shares to your credit in the CDP.  After the shorting, you can buy back anytime you like, one day, ten days or one year or even 10 years later. You decide but you pay interests.

So the above two ways to short are the only ways to short sell shares.  The third method to sell short, is to use an instrument known as CFD.  CFD is just like a share where you buy or sell but you don't need to have shares credited to your acct.  CFDs are just contracts and are somewhat like options.  When you sell a CFD, you actually short the named share linked to the CFD.  Buying CFDs is just the opposite.

Hope this information will give you a good knowledge of shorting.  You can get additional help with your brokers.

Cheers.

Same day shorting should not pose any problem as it is easy for the broker to square off the transactions since you can deliver.

But not if you don't cover on the same day, whereupon, if SGX has to buy-in, you will have to pay penalty as well as $1,000 for inability to deliver on time.

Better to speak to your broker personally for correct info.

In any case, unless you are a 'gu shen' or a stock market medium, why do intra-day shorting at all?...