So I have booked a random weekend to Cambodia during my 1 month visa exempt stay in thailand.

This means the flight out of thailand on my current departure card is now wrong.

Do I try to correct it on the card by writing over the old flight number?

Start again with a new card while keeping the old one with the stamp to explain?

Just say I lost it?

I'm sure it is pretty simple but never really been in the situation before.

Just change the departure card. Nothing to worry about.

Just change the departure card. Nothing to worry about.

+1.

I think that Immigration even doesn't bother to read those cards

The only thing I have ever put on the departure portion of the card has been my name and passport number. Next time just leave the flight number and etc blank and fill it in just before you leave.

Thanks for the answers. I will just write over the old number then.

Not a big deal by the sound of it.

Wasn't aware you could leave it blank.

It seems an odd way of doing things to stamp the departure card on arrival anyway. I don't see the point of it

Maybe to have a record of the entry and exit dates?

I go to a lot of countries and have never been given a card I need to hold onto for the duration of my stay anywhere but here. Most places just stamp in and stamp out in the passport.