What prevents people from buying health insurance and then using it the next day to cover an operation?

For example, Enrolling in health insurance or getting a job with benefits, with the raw intentions of using it to get what you need for a lot less cash. ( Ex. operation or dental work or Xray ) Is there a time limit before you can use the insurance? Is there a contract that keeps you locked in for several years? How could someone turn a 30,000 dollar surgery into a hundred bucks per month with no penalties? Ive got into an argument with a friend about this and he says you can buy it and use it the next day ...
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There may or may not be a time limit. But pre-existing conditions would definitely be excluded. It an individual got started on an insurance plan and a provider submitted a bill soon thereafter, it would get a very good look from the insurance company personnel! If there were any evidence of abuse or fraud it would likely be rejected. Only if it were found to be a total coincidence (like if the insurance started and the next day the person fell and broke their arm) would it be paid. But if it were for, say removal of a tumor they would almost certainly reject the claim and maybe claim fraud if you knew about the condition but did not disclose it beforehand. There is usually no contract.
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