I was thinking about the situation several years ago where a number of pension funds had bought annuities for their retirees at "bargain" rates(for unrealistically low lump sum values) instead of paying the retirement benefits out of the pension trust fund. This may have been due to plan termination and the employer was attempting to recoup as much of the fund as possible for its own purposes. In any case, the insurance company became insolvent and these individuals, who were relying on the insurance company to provide their benefits, had their payments delayed or possibly ultimately reduced. After the annuity purchase, there was no longer PBGC coverage.
In any case, the situation prompted additional pension regulations to be passed that required the plan sponsor to do a more thorough review of the insurance company for solvency problems before purchasing annuities for its retirees or deferred annuities for its vested terminated employees. If not, the sponsor could be found in violation of its fiduciary duty, etc.
Frank