A bug report contains many fields, such as product, component, severity, priority, fixer, operating system (OS), platform, etc., which provide important information for the bug triaging and fixing process. It is important to make sure that bug information is correct since previous studies showed that the wrong assignment of bug report fields could increase the bug fixing time, and even delay the delivery of the software. In this paper, we perform an empirical study on bug report field reassignments in open-source software projects. To better understand why bug report fields are reassigned, we manually collect 99 recent bug reports that had their fields reassigned and emailed their reporters and developers asking why these fields got reassigned. Then, we perform a large-scale empirical study on 8 types of bug report field reassignments in 4 open-source software projects containing a total of 190,558 bug reports. In particular, we investigate 1) the number of bug reports whose fields get reassigned, 2) the difference in bug fixing time between bug reports whose fields get reassigned and those whose fields are not reassigned, 3) the duration a field in a bug report gets reassigned, 4) the number of fields in a bug report that get reassigned, 5) the number of times a field in a bug report gets reassigned, and 6) whether the experience of bug reporters affect the reassignment of bug report fields. We find that a large number (approximately 80%) of bug reports have their fields reassigned, and the bug reports whose fields get reassigned require more time to be fixed than those without field reassignments.