I have find that most of the clients face issue in Paypal IPN and large number of clients has post their issue on freelancing platforms like ODESK, Freelancer etc. I will teach you how to get rid from this issue.
Why such issue come? the simple answer is that due to rounding number this issue occur. On some products you will see PayPal is working fine but at some of the products which have price like $20.9887, In such condition PayPal going to generate error of IPN number or fake IPN or there is something security issue. You can easily solve it by using my below module and file.
1) GOTO app/code/core/Mage/Tax/Model/Calculation.php
Download the zip file from below link and replace this above file.
Place the MageExt folder into the local directory,refresh the cache, now test you issue, it will be solved.
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Contact: message me at email@example.com