Things that will get you an FCC visit are few and far between, but encroaching on or using unapproved frequencies can be one of them, particularly if you are running excessive power and/or causing harmful interference.
The harmful interference part is what is likely to get you, if anything will. I'd wager that 99 percent of those cited by the FCC began with causing some sort of interference - to the degree that they caused multiple agencies to become involved and basically hand the FCC an investigation on a silver platter.
Fines and confiscation are the typical penalties. I know of no one in the last 20 years or more to ever get jail time or any incarceration for a violation. In a world of drug dealers and human trafficking, the odds of jail time are virtually non-existent. However, be advised that using unapproved frequencies typically results in local hams taking notice, investigating and documenting, and then filing multiple complaints, to the point the FCC has everything they need.