Most errors I have seen through plan audits are typical. It’s usually late deferrals or a screw-up on the definition of compensation. Yet there can be some true horror shows out there, I’m talking about catastrophic errors that can lead to disqualification.
The catastrophic errors that will lead to a plan disqualification often result from the failure (usually intentional) of plan sponsors to not include employees for purposes of covering them under a plan. Could be a 401(k) plan with employer contributions, but usually a defined benefit plan. Sometimes the intent was caused by an actuary, other it can be for the advisor or the accountant. The controlled group rules are very basic, so I would be surprised if the lack of coverage wasn’t intentional. There are a lot of great providers out there, but a lot of jokers, that put plan sponsors and their retirement savings at risk.