Distributed software engineering and agility are strongly pushing on today's software industry. Due to inherent incompatibilities, for years, studying Scrum and its application in distributed setups has been subject to theoretical and applied research, and an increasing body of knowledge reports insights into this combination. Through a systematic literature review, this paper contributes a collection of experiences on the application of Scrum to global software engineering (GSE). In total, we identified 40 challenges in 19 categories practitioners face when using Scrum in GSE. Among the challenges, scaling Scrum to GSE and adopting practices accordingly are the most frequently named. Our findings also show that most solution proposals aim at modifying elements of the Scrum core processes. We thus conclude that, even though Scrum allows for extensive modification, Scrum itself represents a barrier for global software engineering, and development teams have to customize Scrum properly to benefit from agile software development in GSE.