Here are some examples:
'Controlling inflation is more important than controlling unemployment.'
I thought the lesson of the 1970s had been well and truly learned. Perhaps not.
' "From each according to his ability, to each according to his need" is a fundamentally good idea. '
It might be a fundamentally nice idea. That doesn't make it a good idea. If you truly believe that it is a good idea, you cannot simultaneously believe in the power of incentives, the tragedy of the commons, the free-rider problem or any of the other insights of economics. In other words, it is possible, strictly, to agree with the statement but to do so logically places you far outside the mainstream. "Intent" is not the same as "outcome" and it is logically inconsistent to agree with statements you believe to have good intent if you know the outcome will be bad.
'Land shouldn't be a commodity to be bought and sold.'
Again, one may not like the idea of property or of property rights. However, to agree with this statement is to call for a complete abandonment of our economic system. That's fine; some people really do believe they know better and think the miracle of the last 500 years (humanity's discovery of how to escape from poverty) could have been achieved better in another manner. But glibly to agree with such a statement without acknowledging its implications is of questionable rigor.
And just in case anybody thinks I'm merely defining my opinion as the truth, here's an example where I do believe opinion comes into it:
'No broadcasting institution, however independent its content, should receive public funding.'
I have strong opinions on this one but I accept that some people genuinely do believe applying a regressive tax on the poor (as happens in the UK) to fund a broadcaster is acceptable. I find such people are immune to (my) argument and have, reluctantly, concluded that this one truly does seem to be a matter of opinion!