With a new technology comes a lot of mistakes as people learn how to navigate that new technology. Below is a community-sourced list of how people have lost their lumens in the past. Avoid these mistakes. Please contribute by editing the list.Edit List
When you create a new Stellar account, be sure to copy down the associated secret key first thing. Do not send lumens to the account until you have confirmed that the secret key has been copied down correctly and unlocks the account.
When sending Lumens to most exchanges, you must include the unique memo that the exchange provides you. Because exchanges hold all of their users’ Lumens in one Stellar account, they need to know which incoming transactions to that account are from you. If you don’t include the proper memo, your Lumens will get lost in the millions of Lumens held on the exchange.
You may have copied down a Stellar address incorrectly, or there may be a man-in-the-middle attack present. If this is the case, you will lose all of the Lumens associated with a transaction. To prevent this from happening, just send a small transaction - less than 1 Lumen if the receiving account already exists - first to confirm that everything is working properly.
Web wallets can be hacked and redirect all transactions occurring through the web wallet to the hacker’s own Stellar account. Unless you have a hardware wallet and confirm the destination account on the device itself, there isn’t much you can do to avoid this outside of not using a web wallet.
To send the entire balance on a Stellar account to an exchange (including the 1 Lumen minimum balance), some people will merge their account with an exchange’s account. Unfortunately, exchange accounts are not set up to catch merge operations and they often won’t recognize these transactions, even if you include the proper memo.
When setting up a multi-sig account, it’s easy to lock yourself out of the account by setting master key weight to 0. If you do this, you will lose all access to that account.
When signing up for an inflation pool, some people will actually send their Lumens to the inflation destination account instead of voting for the inflation destination account.
Some Lumen-holders panic when the price goes below the price they bought at and then they fear-sell.