This is triggered by the recent post by Tyler Cowen and some followups by others. In it Tyler posits three laws: 1) There is something wrong with everything (no slam dunks, and one only understands something if one knows its flaws), 2) There is a literature on everything, and 3) All propositions about real interest rates are wrong. The first clearly contradicts itself, and while most laws may be limited or not universally true, some are truer than others, e.g., round earth model closer to reality than flat earth one. The second is clearly false as some ideas have never even been verbally expressed by anybody, although the real point of this is probably to warn that if somebody thinks they know the full literature on something that has a literature, they probably do not. The final one is probably empirically correct, although properly stated theoretical models contingent on unrealistic assumptions may be correct if their unrealistic assumptions hold. More generally, Tyler warns that we should all be wary of thinking we know too much, which is clearly correct.
In a followup he links to Arnold Kling who poses three laws due to his poli sci prof dad, Merle Kling: 1) Sometimes it's this way, and sometimes it's that way (I can think of some things that are pretty much always one way), 2) The data are insufficient (often the case, but maybe not always), and 3) The methodology is flawed (see 2). He calls these "iron laws of social science," but they do not look any more ironclad than Ricardo's Iron Law of Wages, which depends on some assumptions holding that are in fact not true, such as there being no technological change. More like silly putty laws.
OK, so are there really any fully true laws? Even beyond economics, most "laws" depend on certain assumptions holding for them to hold as well. In some areas, this is not such a big deal, and thus in chemistry, a lot of its laws hold pretty widely. Physics gets a bit iffier, with the good old law of gravity the classic example. Sure enough the equation explains rates of acceleration in a vacuum, but outside of a vacuum, well we even see some things moving away from each other, such as a helium balloon rising away from the earth. Nevertheless, in many hard sciences it is a lot easier to figure out when the necessary assumptions are holding and when they are not so that one can figure out when the supposed laws will apply to the real world or not, even if those assumptions do not always hold everywhere and at all times (and some physics laws hold simply everywhere at all times, as best we know).
When we get to economics, it looks to me that Tyler's first law holds pretty well. Most economics laws do not hold universally. Demand curves do not always slope downwards, even though Misesian a prioristic praxeologists insist so, and socially necessary labor time does not always determine values that equal "natural" long-run prices, even though fundamentalist Marxians might believe so. The basis "law" of supply and demand, although useful for understanding many real world markets, does not always hold in its standard formulation for a long list of reasons. And most of the laws used to study or explicate macroeconomics, such as claims about real interest rates, are by and large wrong or only true in limited cases.
Given all that, I shall note one economics law that I so far do not know of any exceptions to. That is the law of diminishing returns, or diminishing marginal productivity. Keep in mind that its formulation says that "eventually" marginal product of an input will decline, not that it is always declining, and for many agricultural activities many inputs have increasing marginal productivity for awhile. If anybody can think of a real world exception to this law, I would warmly welcome learning of it, but so far, it seems to hold universally.