An Amazon employee earlier today tweeted details about an incident that many suggests could be a sign of a huge privacy bug in the most popular end-to-end encrypted Whatsapp messaging app that could expose some of your secret messages under certain circumstances. According to Abby Fuller, she found some mysterious messages on WhatsApp, not associated with her contacts, immediately after she created a new account with the messaging app on her brand new phone using a new number for the very first time. Fuller believes that the mysteriously appeared content on her new account was the message history associated with the WhatsApp account of the previous owner of the same SIM/mobile number, which WhatsApp pushed to her phone.
Reports suggests that the messages Fuller found on her newly created Whatsapp account were probably only the undelivered messages sent by the contacts of the previous owner after he/she stopped using that SIM number .
Moreover, to prevent your previous messages from landing onto others device, WhatsApp recommends users to either delete their account before stop using a SIM or mitigate the WhatsApp account with "Change number" feature available in the app settings.
You don't need your SIM to keep using your WhatsApp account, once configured on the phone.That means, it is likely possible that the old owner of that SIM was still using his WhatsApp account after dumping the SIM number until Fuller recently configured the same number and verified the account using the OPT received on her phone. So, with high confidence, we can say that the messages appeared on the Fuller phone were only some recently undelivered messages that the old user was supposed to receive.