Learn English – What do you call two words that contradict each other in a sentence

The best way to understand something like this is to replace each occurrence of the repeated word with either a synonym, or something that matches its part of speech:

Just came up with an idea, but the idea I came up with wasn't the idea I believed I came up with.

As you can see, removing the last "I thought" either changes the meaning, or results in a contradiction:

• Just came up with an idea, but the idea I came up with wasn't the idea I believed – inventing an idea you don't believe is quite different from misremembering what idea you invented.

• Just came up with an idea, but the idea I came up with wasn't the idea I came up with – this is a contradiction, and doesn't make a lot of sense. (Well, not that the original makes a whole lot of sense, either.)

I don't know if there's an official term for this sort of phrase. Depending on the repeated word, it could be considered a tongue-twister. For the famous buffalo example mentioned by MrHen in the comments, Wikipedia just says it's "an example of how homonyms and homophones can be used to create complicated linguistic constructs."

"How I love the country life" is a pun, because in that context 'country life' has two meanings — life in the country and the margarine brand.

"Those two pears are a pear of green balls" doesn't look like a pun to me, as nothing in there has two meanings.