In "Word Focus" Glenn McClure writes:

“The kind of stumbling block or hindrance which Paul had in mind is the setting of an example which might lead another into sin. A Christian ‘stumbles' if, following the example of a more emancipated Christian, he does something of which his own conscience does not really approve. In consequence his spiritual life will suffer grave injury” (Bruce, Romans). Thus, the stronger Christian should resolve to live out his Christian experience so it would not be an occasion of leading his weaker brother to sin, whether by example or by a severe and harsh judgment which would provoke him to anger, or create jealousies, envyings, or suspicions. There is no allowance for indifference, for the influence which the believer's conduct may have on another believer should be exercised in consideration which is due to them. The searchlight of divine love should be turned upon our own hearts in order to determine whether we are stepping stones or stumbling blocks.

Source: Studies in Romans: Adult Teacher's Insights, page 76.