[2461Badugi]

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I think he looks down at a four straight board, looks at his strong one pair hands (majority of his range I put him on, obviously busted draws, hit draws, two pairs, trips could all be here from someone like this) and gives up.

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It's not so much gives up as decides he wants a showdown. It's very hard to value bet one pair into this board, and he doesn't want to let you use the four-straight to bluff him off a reasonably-sized pot.

You have to find some sort of a bet here. What changed your mind from when you planned to bet 500 to when you actually checked?