Your complaint is a valid one. Last month a good customer of mine wrote me a check and dated it July 22, 2012. I chalked it up to him being in a hurry. I called him and he told me to change the date and put his initials on there and then cash it. Which I did.
The problem is, I try to build trust with customers (which are all word of mouth referrals). One of those ways is trusting that they wrote out the check properly or gave me the right amount of cash. So when they hand it to me, I put it right into my pocket while wrapping up our conversation.
Also last month the guy meant to give me $130 and he only gave me $105. Instead of counting it, I just put it into my pocket. I'm not going to chase the guy down for $20 but next time I do work for him I am going to be very deliberate when I count it and mention that he shorted me $20 the last time. If that's the last time I work for him, so be it.