1. Delete a thread with a liked post in it.
2. Using the inline mod tools select the thread and select the undelete option. This causes the undelete form to be submitted however if you hit the go button as well (you can press it multiple times) it will send the form more than once.
3. Once it has been submitted view the user's likes again and they will be increased.
In case of a large thread or slow server this can take some time, causing the button to be hit multiple times as you wait for it to respond if you are impatient.
The mod logs will also look something like this.