Preaching a bad homily is something to fret over. It is a sin. And, as such, should be taken seriously.
When God's Word goes forth from the mouth of the Pastor (in this case, from the mouth of the Pastor through the seminarian) it isn't idle. It has power to kill and make alive. When you are not clear, you can confuse the people and confound law and Gospel for them. Thus, effectively preaching them into hell without ever preaching them back into heaven. Or you may effectively create secure sinners and thus lead them by the hand to hell in their smug state.
I preached a bad homily yesterday. I sinned and needed forgiveness. I should have been clearer. I should have better prepared my sermon. But I didn't, and there is no second chance for that moment in time when God was breaking once again into the lives of sinful humanity.
Always be bold when preaching. Then again, always remember that when you fail to preach rightly, as God has said you must do, trust in the promise of forgiveness of sins which Christ gives to you in the Supper you'll receive in a few short minutes later.