The Black-Scholes model is derived under the assumption that hedging is done instantaneously. In practice, there is a "small" time that elapses between buying or selling the option and hedging using the underlying asset. Under the following assumptions used in the standard Black-Scholes analysis, the value of the option will depend only on the price of the underlying asset S, time t and on other variables assumed constants. These assumptions or "ideal conditions" as expressed by Black-Scholes are the following.
"The extended Black-Scholes model with-lags-and "hedging errors","
International Journal of Banking and Finance: Vol. 1
, Article 6.
Available at: http://epublications.bond.edu.au/ijbf/vol1/iss2/6