You're going to have to clarify what jurisdiction, since USA law is going to be vastly different than EU law, in the realms of product, medical devices, and public accommodations liability.

But if we did examine the USA, then we can find some generalized rules. For product liability -- the responsibility of manufacturers and distributors of a tangible object -- strict liability will lay when a product has an inherent defect (meaning it didn't become defective after the initial sale) and this defect causes some sort of injury. Although this criteria doesn't depend on the frequency of injuries, if a product is accumulating a body count, that's usually a good sign that there's a defect. Causality is also important to establish, as well as any mitigations that may have existed. On this front, a manufacturer might argue that the warnings in the instruction manual specifically advised against diving headlong into a 30 cm deep swimming pool. And although warning consumers to not do something may be somewhat effective at discharging liability, warnings alone do not prevent someone from trying a lawsuit anyway; the popular wisdom that the "pages of warnings" in manuals are written by lawyers is only partly true, since most manufacturer prefer repeat business by customers that are still alive.

Medical product liability is similar, but slightly different because medical products are built for a specific purpose but a doctor can instruct a patient to use it differently, if medically appropriate. If not used as instructed by the manufacturer, the manufacturer is usually off the hook, but the doctor might be liable for medical malpractice. Maybe. Doctor liability in the USA is framed within a "duty of care", meaning that the doctor takes on a responsibility to act with a reasonable degree of skill and competency. The "standard of care" idea is related, in that it sets the floor for what is reasonable for all doctors. It is, for example, grossly negligent to a drunk doctor to examine a patient. Harms from such negligence can be litigated through a malpractice suit. But this doesn't mean all harm is actionable. A successful appendectomy that results in blood sepsis is always going to be a possibility, even with the best infection controls in place. If all the staff discharged their duties within their training, then negligence does not attach. Also, malpractice is not something which can be waived, because even if a patient doesn't sue, a doctor's medical license can be suspended. Whereas the risks of a surgery can be described in detail to a patient, for informed consent.

Finally, public accommodations law sets the floor for how public and private businesses conduct themselves if they provide goods or services to the general public. Very prominently in this realm are accessibility requirements, which are rules that assure the disabled will not have undue burdens that able-bodied people wouldn't face. The Americans with Disabilities Act (ADA) provides for very stiff fines for non-compliance, and because its objective was to set the standard, there is no provision for a "fix it ticket" approach for enforcement. That is to say, the ADA does not allow business owners to wait until a wheelchair user makes a complaint; they must follow the standard from day 1.

No doubt there is abuse of the liability laws -- there's nothing more American than filing "ambitious" lawsuits -- and this is just a brief (and uncited, '"from the hip") summary of possible areas of law that might answer your question. But I hope it gives you an idea of why a warning or sticker or sign might incur liability. Or at the very least, an unexpected lawsuit from left-field.