Multi-user Extended Reality (XR) systems enable transformative shared experiences but introduce unique software defects that compromise user experience. Understanding software defects in multi-user XR systems is crucial for enhancing system reliability, yet remains underexplored. To fill the gap, this paper presents the first large-scale empirical study of multi-user XR defects, analyzing 2,649 real-world bug reports from diverse sources, including developer forums, GitHub repositories, and app reviews on mainstream XR app stores. Through rigorous qualitative analysis using iterative open coding, we develop a comprehensive taxonomy that classifies multi-user XR bugs along three dimensions: Symptom Manifestation, Root Cause Origin, and Consequence Severity. Our findings reveal that synchronization inconsistencies and avatar-related anomalies are the most prevalent symptoms, while network/synchronization logic defects and session management flaws emerge as dominant root causes. Critically, over 34% of analyzed bugs lead to severe consequences that fundamentally break the shared experience, including system crashes, persistent disconnections, and complete interaction breakdowns, etc. We also identify concerning privacy and health implications unique to multi-user XR contexts. Based on our findings of defect analysis, we provide actionable recommendations for developers, platform vendors, and researchers. Our results demonstrate that multi-user XR systems face distinct challenges at the intersection of distributed systems, real-time 3D interaction, and immersive experiences, necessitating specialized approaches to testing, debugging, and quality assurance. Our datasets and results are released to facilitate follow-up studies: https://sites.google.com/view/multi-user-xr-study.