If you have thought prepaid connection is safe because the connection is automatically disabled when you run into negative balance, you are wrong.
Hear my story:

I recharged my photon+ prepaid around June 10 by 4GB by paying Rs. 1000 at an outlet.

When the expiry date was nearing in July 10, I wanted to use up all the currency left, so I started using photon more and more and I realized that I was not at all getting any SMSs from Photon about the balance. I used to get a few SMSs a day earlier.

Anyway, I never worried as I was expecting the photon will stop working once I cross the usage limit.

On July 11th, it stopped working.
I charged for Rs1350 on July 12th online.
Now when I checked the balance online, this is what I See:

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Destination Number Date of Call Time of Call All Charges (Rs.) Call Duration (Sec.)
0 12-JUL-2010 10:56:48 AM 586.0 0
0 12-JUL-2010 10:56:47 AM 2952.99 0
0 12-JUL-2010 10:53:30 AM 10240.0 0
0 12-JUL-2010 04:15:17 PM 2.0 0
0 12-JUL-2010 04:14:51 PM 2.0 0
0 09-JUL-2010 09:15:20 PM 165.01 0
0 09-JUL-2010 05:51:31 PM 471.0 0
0 09-JUL-2010 05:45:09 PM 41.0 0
0 09-JUL-2010 03:56:58 PM 267.0 0
0 09-JUL-2010 03:46:34 PM 5.0 0
==================================================
It is beyond my brains to understand how can I use 10240 units at 10:53 and then
2952 at 10:56

It says my balance is zero :-(
Worried, I called the support.
A guy said someone will call me back.
Nothing happened for a day.
I call back today, the Lady who attended me said that I had a negative balance of 4GB.
And there was a technical glitch at tata side which could not disable my card when it went into negative balance. (So they decided to recover the leaked bandwidth by penalizing me when I charged next time! It is like when I goto a gas station and ask for 10Ltrs of gas, the attendant fills 20Ltrs and ask me to cuff up more money...)

Well we buy prepaid hoping that we will never get into this problem.
I think tata deliberately program it that way to cheat their customers.
If I had charged for Rs. 1000, which I normally do, I would have left with no balance,
whole 1000 rupees would have gone.

Below are conversations between me and photon support staff.
1. http://www.youtube.com/watch?v=AioRM6rUJF8
2. http://www.youtube.com/watch?v=cXPgAL_Uf-w