What circumstances could lead to a Thai Immigration official questioning my motives during a land-based border run for a tourist visa?

On a land-based border run (for a new 30-day tourist visa-exempt stamp), under what circumstances might a Thai Immigration official question our plans/motives? I’ve read that officials have sometimes suspected a tourist is gaming the system and then disallowed return entry, but I can’t recall any particulars. As long as it’s the first or second land-based border run in a single calendar year, and you’ve not overstayed, when—if at all—could a tourist be in jeopardy of being refused re-entry? Thanks in advance.