SP - I explained below what thing I think is coming. I've never understood it to mean YOU'RE going to have to come up with another idea, I've always read it as EVENTS are going to make you change your mind - i.e. "something's going to happen, the opposite of what you believe".
I suppose I am applying Occam's Razor. There are two possibilities:
a) that it is genuinely 'think', because people mean "if you believe that [wrong] thing to be true, then you're going to have to sit down and have another think about that and come up with another idea"
or
b) People are generally ignorant, and over the years the use of the word 'thing' has become corrupted through laziness and estuary-speak to 'think'. As with 'nuffink'.
I know which I consider to be the one involving the fewer unnecessary complexities, and therefore I go with that.
(From the very first episode, RB - Doctor to Ian!)