My manager sometimes makes commitments to stakeholders on the spot when he agrees to take on the work, without consulting with the developer (me, in these cases). Often the requirements are not even clear yet.

It's not comfortable to do, but I will jump in and veto his commitments right there if they are clearly not attainable.

The stakeholders tend to be accepting of the fact that things are more complex to implement than just: "make the software do X", but my (technical) manager always seems a bit baffled. In the long run, I know he values this about me, but it frustrates me to no end.