I have seen many discussions of authorship conflicts, which usually relate to a substantial contribution not being recognised, or a more senior supervisor insisting on co-authorship.
However, I am in the opposite situation: I have been offered co-authorship on a paper (currently in preparation for journal submission), and my contribution so far was very small. I am aware of the ICMJE guidelines on authorship and in truth my contribution to the acquisition/analysis of data probably wasn't "substantial".
Ethically, the right thing to do is probably to decline co-authorship.
(In fact, I am included on a previously published paper to which I made no contribution. My PI included me: he argued that I contribute to the group's work on this subject as a whole. But it does not feel right!)
But, I feel that many people would not decline... therefore if I decline, I will have one fewer paper on my CV than somebody who would accept. In the long run, presumably this puts me at a disadvantage to people who accept such co-authorships, e.g. this could include people I will be competing against the next time I apply for a job (I am an early postdoc).
Should I accept co-authorship and live with the slightly guilty feeling? Am I disadvantaged if I decline?
Note: I am particularly interested in the career implications of this decision/future decisions like this, and what is the "norm" - rather than answers that just apply the more "theoretical" criteria for authorship (such as ICMJE).