In Kanchanaburi, about seven years ago, I went to immigration on Monday with a visa that had expired on Sunday.

Immigration extended the visa and did not charge for one day's overstay.

Is this standard practice or was it a lucky one-off ?

What kind of visa?

That is standerd practice. If the permission to stay expires on a day immigration is closed you cna go for an extension on the first day they are open again without being charged overstay.

Do note that you can always apply for an extension a few weeks early. It will still start on the day the permission to stay ends, so you don't lose any days.

That is standerd practice. If the permission to stay expires on a day immigration is closed you cna go for an extension on the first day they are open again without being charged overstay.

Do note that you can always apply for an extension a few weeks early. It will still start on the day the permission to stay ends, so you don't lose any days.

Not much to add, thanks Mario !