As mentioned on here elsewhere, there are certain sins which, due to their especially grave nature, are reserved to Rome such that a priest alone is not able to administer absolution. These sins would include, among others, the desecration of the Eucharist, abuse of a minor by a priest, breaking the seal of confession, etc.
One of the oddest sins on the list can be phrased something to this effect: "The sin of a priest soliciting sexual activity from the confessional." Basically, if a priest solicits sex from the confessional he can't go to another priest for absolution - it must be handled by Rome itself.
Canon lawyer Ed Peters recently had a very fascinating post, however, that shows that this sin also has always also been about priests who in any way encourage unchaste sexual activity, even if the activity is not with the priest himself.
Basically what this means is that if a priest were to encourage adultery, pornography, masturbation, contraception, etc. for a penitent, then that priest would be guilty of a sin that another priest can not give absolution for.
A priest soliciting sex from a penitent is rare and obviously scandalous. MUCH more frequent, however, are the cases of priests who offer sexual advice which is nothing short of encouraging sexual sin.
Peters makes the case that only those offenses involving the priest himself should be reserved to Rome, but that other violations of the command ought to be reserved to the diocesan level at least.
You can read Dr. Peter's post here.
Pray for our priests that they may be kept free from these types of grave sins.