bitcoin transactions are only irreversible AFTER confirmation.

writing a transaction. you can validate the transaction by checking the inputs and checking the balance is available to spend and the signature belongs to the inputs. but until the transaction is confirmed into a block you dont yet have the funds to spend yourself and the transaction can be rejected.

there are a few ways to write a transaction announcing you as the destination. making you see the unconfirmed transaction. but then write another transaction using the same original input funds to move the funds elsewhere and getting that transaction accepted into a block before the one you were hoping for. thus making the one you were hoping for get rejected.

so until the transaction you were hoping for is confirmed (irreversible) dont trust it. especially ones with insufficient fee