And, suppose you don't pay the penalty and the IRS assesses the amount due. This isn't as severe as an ordinary unpaid tax either:
The bill signed into law (on page 131) specifically prohibits the IRS from using the liens and levies commonly used to collect money owed by delinquent taxpayers, and rules out any criminal penalties for individuals who refuse to pay the tax or those who don’t obtain coverage.
Nobody goes to jail for not complying, nobody is subjected to tax liens for not complying, nobody is forced to actually apply for and obtain health insurance, and nobody is forced to pay the actual full cost of obtaining health insurance. It is just a modest money fine that is enforced like any other debt. It is actually less onerous than an excise tax on people who fail to provide health insurance for themselves.
Thus, if you file a tax return without the proper documentation to show that you have coverage, and can't provide it when the IRS audits you, and no exception applies then you owe the penalty, which the IRS may only enforce like any other civil debt, with a lawsuit to obtain a civil judgment in that amount.
In other words, the only penalty for failure to comply with the health care mandate is the equivalent of a parking ticket, or even closer to this case, a public health user's fee similar to a mandatory trash collection bill sent only to people who don't privately obtain trash collection services.
And, if you do comply, you get considerable tax benefits. This is 95% carrot and 5% stick.
It is hard to imagine a mandate more mild.