when and where the error"input string is not in correct format" will exist?

When you are tring to convert a string into a type which it cannot be converted to. For ex:

string word = "Hi";
int result = Convert.ToInt32(word);

Or when you are simply assigning a value that the certain type does not support.
For ex:

Label1.Text = "ABCDE";
double result = Label1.Text

Regards,
Eduard

Read The following:
Error: Input string was not in a correct format.
System.FormatException occurs when attempting to convert a numeric string to a numeric data type