If you have been dishonest with your former employer, this is very serious. The employer cannot be expected to give you several warning for this type of behavior. Theft of company time by dishonestly changing time sheets is a common one. Most employees know that this will indeed get you to termination quicker than some of the other charges against you. If your former charges you with dishonesty, the only way to defend it is that you believe that it was misunderstood and not taken in the context that you wanted. They may have not heard you correctly or was focused on something else that took their attention away from what you were saying.
Another defense of this charge could be that you were joking or straining your voice over other sounds to make sure you were heard. You can state that it was just an error and that you did not mean the action or the statement maliciously. Explain that you were doing the best you could do within the circumstances that were presented to you. Your previous employer may have been very critical of your work and looking for any slight to target you.