This happens most often when the call is placed with a cell phone.  Audio Banking expects callers to enter their PIN and then press the # key.  However, some users don't realize that they need to press the # key when they're done.  Audio Banking accommodates these users by measuring the amount of time between key presses.  If a certain amount of time has passed since the last key was pressed, Audio Banking guesses that the user is done.  The default delay that Audio Banking uses is 3 seconds.  Cell phone users are often distracted (by driving, for example) or are working with very small keypads on their phones, so the time that passes between key presses could be greater than 3 seconds.

Here's an example:  Tom is driving down the road and decides to call Audio Banking to check his account balance.  Tom's PIN is 1234.  Tom enters "12" on his cell phone but then has to dodge a large pothole in the road.  4 seconds pass, and Tom enters "34" and Audio Banking responds by re-prompting Tom for his PIN.  What has happened behind the scenes is that Audio Banking considered "12" to be one PIN attempt since the 3-second delay had been exceeded.  Audio Banking tried to re-prompt Tom for his PIN, but Tom interrupted the message by pressing "34."  Audio Banking considered this a second PIN attempt and re-prompted Tom for his PIN because "34" isn't 4 digits long.

The default delay can be increased to accommodate these situations.  However, this must be balanced against the fact that if the delay is too long, users who simply forget to press the # sign will get frustrated waiting for the system to continue to the next step.