united-states
Using LLC assets for personal use when you are the sole owner of the LLC and it is disregarded as an entity for federal income tax purposes is not so much "illegal" as it is a choice that has legal consequences.
It may mean that you can't deduct some LLC expenses as business expenses, and may destroy the value of the LLC in protecting you from liability for LLC debts. But it isn't a crime and the only penalty you pay are the consequences set forth below.
Federal Tax Purposes
For tax purposes, as a disregarded entity, the issue is that you should not take a tax deduction as a business expense for LLC purchases which are used for personal purposes.
For example, if the LLC buys 1,000ft of CAT6 (network cable), and you cut yourself a cable for personal use, the portion of the cost of the 1000 ft of CAT6 cable which is used for personal use should not be deducted as a business expense.
This is the same analysis that applies to distinguishing business expenses from personal expenses in a sole proprietorship that isn't an LLC.
How single member LLCs are treated for state tax purposes varies considerably. Many states follow the federal model, and many other states do not.
Debtor-Creditor Purposes
For state law debtor-creditor law purposes, it is important to keep LLC property distinct from personal property.
At a minimum, distributions from an LLC entity to you as an individual are transfers made without receiving substantially equivalent value are fraudulent transfers if the LLC is insolvent. This means that if the LLC's assets are insufficient to pay its debts, and a creditor asserts that you made a fraudulent transfer, that you have to repay the distributions made to you until the creditor's claims are satisfied (and in some states, you owe a penalty as well).
If you don't carefully account for distributions, including personal use of LLC assets for personal use, in the books and records of the company in a contemporaneous manner, then you have destroyed the limited liability protections of the LLC entirely in most cases, either through "piercing the corporate veil" or through the "alter ego doctrine". Either way, all of your personal assets become subject to all debts and liabilities of the LLC.
Using LLC assets for personal use without carefully accounting for this as distributions to you in the books and records of the LLC is by far the most common fact pattern in which the corporate veil is pierced and an owner of the LLC becomes subject to its debts.
And, because the whole point of having an LLC is to secure limited liability protection, you have basically rendered your LLC worthless.
On the other hand, if the LLC has sufficient assets in its own name to pay its debts, or somehow or other, the LLCs debts simply get paid as agreed, this doesn't really matter. You have essentially, an expensive and overcomplicated trade name and as long as the creditors get paid in full, it doesn't matter.
If the LLC has few business dealings and few contractual debts, this may not be a big deal so far as the contractual debts are concerned, but there is still the possibility of tort liability (e.g. defamation or personal injury liability).
But, an LLC only protects you from vicarious tort liability (e.g. liability of the LLC for civil wrongs committed by its employees and agents). An LLC doesn't protect your personal assets from liability for civil wrongs that you personally participate in committing, which is pretty much all of them if you are the sole owner-employee-agent-officer/manager of the company.
And, of course, losing what LLC protections from tort liability that you do have doesn't matter much (although it may have a slight impact on settlement negotiations) if you have sufficient insurance against it to cover any claims against you.
In other words, using limited liability company assets for personal use sometimes deprives you only of protections from creditors that you didn't need anyway. If it does, and the LLC is also a disregarded entity for tax purposes, you aren't getting any benefit from having an LLC, but it isn't "illegal" to have an LLC that provides you with no benefits (apart from a modest penalty for fraudulent transfers from the LLC in some U.S. states).
