In front of me while doing a land border run from Pedang Bessar a Chinese guy had a little difficulty as the immigration officer could not find the entry stamp into Malaysia. He had left Malaysia seconds before and seeking the entry stamp into Thailand.
He entered Malaysia correctly and legally. The IO simply couldn't find the stamp. They later found it and he was permitted entry to Thailand.
As as US Citizen you can get 2x Non visa entries, 30 days extendable for 30 days plus at least one or two SETV which will last 3 months each. gets you 10 months of the year...