die Katze im Sack kaufen
To buy the cat in the sack. Duden defines it as to buy something without first making sure of its quality or usefulness. It also means to accept something one has not examined.
das Kleingedruckte
The small print. The conditions of a contract printed in small type.
die Rechnung ohne den Wirt machen
To make the bill without the innkeeper. To make plans without counting on a person or a fact that decides the outcome.