In online trading accounts, the margining in software is really bad. There are lots of bugs. Or the software is extremely stupid not taking care of trading conditions.
Some examples :
Take the prev. example by SavantGarde, and, say you have a margin of 50k and 1 lot of Nifty Futures require 35k. If you are long with 1 contract, you cannot place an order for Stop & Reverse, meaning you cannot sell 2 contracts! You need to sell 1 contract and only after execution of the sell order you can place an order for selling the next contract!
Let us say, you place a stoploss sell order for selling 1 Nifty Futures contract (same conditions as above), then your margin is apportioned for the stoploss order, and, you cannot buy a contract now! The way it is, you have to place a buy order, and, only after it gets executed you can place a SL order!
Let us say, you have a margin of Rs.1 lakh, and, you place a sell order for 1 Nifty contract, and, is not executed yet. Now you buy 1 Nifty contract. You cannot buy one more contract as the sell order and bought position have consumed 70k, and, the balance 30k is not sufficient for buying one more contract. If you cancel the first sell order, and place a sell order again, then you can buy one contract, and, you will still have 30k margin left!
Everyone will understand that these are really absurd and stupid, but that is the way it is now!
These are just the samples, and, is really stupid. If the software is so terrible and cant calculate margins for just future contracts buy/sell, imagine the plight if you are hedging with options! For a simple example, You need double the margin for buying 1 Nifty Futures contract and selling 1 Nifty Call! These guys never understand!
These problems are there with Reliance Money, and, I do not know about others. So when you do your homework on margins & backtesting, plan sufficient margins accordingly, and, check with your broker or online brokerage about the margining system! You WILL be surprised they would not know and will get back to you shortly!