In such case, at land border, going OUT/IN without having the other side visa and entry/exit stamp, Thai Immigration in Bangkok or elsewhere would consider this an illegal entry. You must have the other side visa and entry exit stamp.
if he just had a non immigrant visa they would give him this stamp, of if he already had a 30day extension on an tourist visa or visa exempt they will give this denial stamp stamp too.
Re your option for a stay up to September. You can do OUT/IN at airport for 30days, get a 30day extension, already 60days. Or do a border bounce to another country, also give you 30days, extendable by another 30days. Or get a Tourist Visa, 60days which too is extendable by further 30days, total 90days.
Robert Stewart , do not overstay. Just go closest Immigration and extend 30 day for THB 1,900.-. Duncan Hockey we are here for visa advise. I'll turn comments off for this topic. It have been answered.
my husband was in Savannakhet last week and The Consular Officer were asking for updated bank statement from everyone. Some people with a visa history required all three items, newcomers only bank statement.