I take your point about vowel clipping and pronunciation, but I think it's possible that the special rhythm that's given to the phrase when you say the whole thing - especially if it's embedded in other speech and could be speeded up - could make that confusion possible again. The phrase sort of takes a poetry/chanting kind of rhythm when you say it. I'm sure it's often pronounced like "thing" by the speakers I'm familiar with, but I'm not convinced that means it is. I know some of those speakers would describe it as "think". And the polls mentioned in that link above and the others I found with Google seem to indicate a near-even split of opinion in the American samples as well.