Out-of-sequence (OOS) construction is regarded as one of the most significant factors contributing to construction inefficiencies and loss of labor productivity. Nevertheless, no prior research efforts have attempted to study the impacts of OOS work on productivity, cost, or even schedule. The goal of this paper is to identify and study the causes and early warning signs of OOS work. Using an expert-based analytic approach, the authors formed an industry panel of 13 construction professionals to gather the basic information related to OOS work and consequently surveyed 88 other industry respondents. The panel as well as the survey enabled the authors to (1) recognize the extent of OOS work, (2) identify 88 causes of OOS work and quantify their characteristics, (3) identify 54 early warning signs of OOS work and investigate their relationship with the occurrence of OOS events, (4) classify the causes and early warning signs of OOS into 11 different categories, and (5) investigate the impacts of OOS work on project performance in terms of productivity, schedule, cost, quality, and safety. Among many important and interesting results, it was clear that late design deliverables represented the most important factor causing OOS work, and late start of precommissioning activities was the most highly ranked early warning sign for OOS work. This research adds to the body of knowledge by providing unprecedented knowledge on why OOS work occurs and by providing a suite of practices that contribute most to OOS work in construction projects. This paper fills a significant gap in the literature and contributes in assisting project participants in better understanding the overall industry perception on the different causes, early warning signs, and impacts of OOS work.