Suppose I enter Thailand by air with an Italian passport, under the visa exemption (which guarantees me 30 days in Thailand). Then I want to travel to Laos by land and come back to Thailand by land after a couple of days.
You'll just get a new 30-day stay. Note that the rules have changed. There used to be two groups of countries that got 30 days: a few countries (including my own, France, which is how I know), called 30/30, whose citizens got 30 days for arrival by air, land or sea; and quite a few more countries, called 30/15, whose citizens would get 30 days for arrivals by air, and 15 for arrivals by land or sea.
This has changed, late 2016 if memory serves. The two groups have been merged into one. We all get 30 days, by any means of transportation. But, for arrivals by land or sea, we can get a visa waiver twice a year only. It's not anymore about 30 days or 15 days, but about the frequency. More than twice a year (and it's unclear whether it's a calendar year or a rolling 365 days), and you don't get in.
I'm not sure how well implemented this is, Thailand being, well, Thailand, but I'm pretty sure they are, or will be, trying to enforce the system, even if selectively, and in stages.
I haven't tried my luck yet. I go every month to Thailand, sometimes twice a month, and sometimes cross into a neighbouring country and back within 24 to 72 hours. I have kept my exits and entries by land (the legal ones anyway) to a minimum. Most of my entries in the last 18 months were done by air, to be on the safe side, or by crossing land borders (Laos, Myanmar) at illegal but very common border crossings.
So if you do a Laos visa run just the once, you'll be fine. Just don't do it too often by land. If you want to do regular visa runs, fly to Vientiane, Phnom Pen, etc…
