Burgess J.W. Raby & William L. Raby have published Prohibited Transactions, Qualified Plans, and "Reasonable Cause," also available on the Tax Analysts web site at 2004 TNT 247-27.  Here is the Introduction:

Having a pot of money readily available to help take advantage of business opportunities, keep the business afloat, or meet pressing personal needs can be a temptation impossible to resist. Yet that is the situation when the small-business owner is the administrator, the major beneficiary, and the trustee of a qualified plan. Taking advantage of that situation raises the possibility of excise taxes on prohibited transactions and penalties for failure to file excise tax returns and pay the tax. Sometimes, as in the first case discussed below, the violation of the prohibited transactions rule is blatant and the individual involved seems to have no defense, but sometimes, as in the second and third cases, it is less clear that what was done should be penalized.

• Ralf Zacky v. Commissioner, T.C. Memo. 2004-130
• Joseph R. Rollins v. Commissioner, T.C. Memo. 2004-60
• Flahertys Arden Bowl Inc. v. Commissioner, 115 T.C. 269 (2000)