For example, lets say you make a mistake in your test so you are checking (result = expected_result), which is always true, instead of (result == expected_result). Now when you write your code, you run the test and it passes.
In this case your code may or may not be correct, and the test, which contains a bug, does not catch it. But the bug is not a fundamental misunderstanding of the problem, rather a simple mistake in writing the test. Following strict TDD doesn't prevent this.
Actually, following strict TDD does prevent this.
You must see a test fail before you make it pass. It goes "Red -> Green -> Refactor" not just "Green -> Refactor". Even if a valid test case passes on the first time, one should modify the test (or the code) in a known way to cause an expected failure.
Your premise (tests are only as correct as the spec of initial assumptions) is correct, but your supporting example is not.