@Martin ********
it's not a paper form, so it's not possible to "not have it". From reading the news article the provided link points to (you're welcome to read it yourself), there will be "help points" at airports upon arrival, they're there for "exceptional circumstances only". Reading into that, I don't think forgetting is an 'exceptional circumstance' and, just my thoughts, one 'could' be refused entry. Like everything else with Thailand, it may well be up to the Immigration Officer you're in front of.