For example, while an increase in the outright purchase of government bonds by a central bank might have some effects, if it induced market uncertainty with respect to future fiscal discipline, it would lead to an interest rate rise by way of increase in risk premiums and thus entail a risk of having an adverse impact on the economy.
In fact, when looking at the recent development of long-term interest rates, they often sensitively responded to views concerning the future issuance of government bonds such that they rose recently reflecting budget discussions.