As with all crimes there is a huge difference between violating the law and being caught.
Probably the most problematic and easiest SEC rule for a small investor to break is insider trading. In most firms employees are informed of changes to the firm's financial situation or new clients that are close to signing in town halls before they are released to the market and many if not most employees of any company will have an idea of how the firm is performing from their day-to-day work. This will result in most employees having non-public information on their company. If that information contains information that is not priced into the stock price it is immediately material. Trading on your own private account on material non-public information about your own company is insider trading. It doesn't matter how big your account.
Since your broker likely doesn't know who your employer is (mine doesn't) especially if you changed jobs since your last KYC update it is very easy to do this and not get caught. Who realistically knows and cares enough to raise this with the SEC? That said many people still get caught insider trading in this way and it is common for people in a company who are likely to have such information to be closely monitored. In spite of the fact that I am closely monitored thanks to the information I have my partner isn't so she could break the regulations on insider information that I have and probably not get flagged.
Clearly it is very easy for someone to intentionally violate the rules from a retail brokerage account in this way. It is less clear whether they would be caught breaking the law.
I have worked in the industry for many years and worked on compliance monitoring until a short time ago. I can tell you that we don't just monitor for brokers' clients actually breaking the law; we also monitor for any suspicious activity that could indicate a rules breach. The closer you get to demonstrably breaking the law and the more significant or frequent you are "lucky", the more likely we are to pass your details on to regulators.
I chose insider trading because it is simple to explain how we can monitor for it; we have a trades feed, a news feed and a market data feed and when the price moves we look back for a piece of news that might have caused it. We then look at all trading in the few days before the announcement that might indicate a trader knowing the news ahead of time. The more times and the more profit (in % terms) a trader is making on these the more likely it is that it will be passed to the SEC. Note that this doesn't require us to know anything about the person trading or have any idea how or why they might have the insider information, it is enough that they are trading in advance of the news. Note that in order to profit on insider trading you normally have to buy or sell and hold for a few days until the news appears.
All of this monitoring is automatic now and once your account is flagged for a potential breach of one rule the analysts will look at all of the potential breaches for that account in the system to get an idea of the trader's pattern of trading and whether it is likely enough that they are breaking the rules to raise it with regulators.
In case you were wondering there are also some very powerful pump & dump analytics available which might well flag you.
Addendum for posterity:
The following is the SEC definition of nonpublic information as a reference to my quick example:
Nonpublic, or inside, information about the Company that is not known to the investing public may include, among other things, strategic plans; significant capital investment plans; negotiations concerning acquisitions or dispositions; major new contracts (or the loss of a major contract); other favorable or unfavorable business or financial developments, projections or prospects; a change in control or a significant change in management; impending securities splits, securities dividends or changes in dividends to be paid; a call of securities for redemption; and, most frequently, financial results.