From what I understand, the FASB is (relatively) fast to respond to
such exploits. However, is it legal to take advantage of one prior to
it being fixed? Technically, doing so neither violates GAAP nor
falsifies financial reports; however, it still purposefully misleads
potential investors.
GAAP (the Generally Accepted Accounting Practices promulgated by the private, non-profit Financial Accounting Standard's Board (FASB) which is controlled by a Certified Public Accountant (CPA) professional organization), doesn't really have loopholes, because of the way it is structured hierarchically.
Essentially, there is a top level conceptual description and then a layer or two of more specific guidance beneath it. But, there is also an ongoing general obligation that applies to everything to not be misleading or confusing, even if no specific provision of GAAP says so.
If just including a raw number in a standard description accounting entry (even if the number is the correct one under GAAP) is misleading or leads to ambiguity, there is an affirmative obligation under GAAP to footnote the entry in question to clarify and explain what the financial statements say to resolve the otherwise misleading or ambiguous character of an entry (or to clarify the misleading or confusing effect of a lack of an entry, even if the omission appears to be authorized by other specific GAAP provisions).
As noted in another answer by timuzhti, Securities and Exchange Commission (SEC) Rule 10b-5, imposes liability in general for making misleading statements in connection with the sale of securities.
Also, liability is imposed pursuant to other statutory provisions of the Securities Act of 1933, and the Securities Exchange Act of 1934, if the misleading accounting statement is part of the SEC required reports of a publicly held company subject to SEC jurisdiction. The statutory provision dispense with some of the elements that would otherwise have to be proved in a 10b-5 or common law fraud case (e.g. proof that the statement was made by the issuer, intent that the statement made be relied upon, and reasonable reliance on the statement by the person harmed). Still proof that the statement is misleading, that it is material, that it caused damages, and some level of scienter (although in practice, not as great as for common law fraud) is still required.
