Software build process translates source codes into executable programs, packages the programs, generates documents, and distributes products. In this paper, we perform an empirical study to characterize build process bugs. We analyze bugs in build process in 5 open-source systems under Apache namely CXF, Camel, Felix, Struts, and Tuscany. We compare build process bugs and other bugs across 3 different dimensions, i.e., bug severity, bug fix time, and the number of files modified to fix a bug. Our results show that the fraction of build process bugs which are above major severity level is lower than that of other bugs. However, the time effort required to fix a build process bug is around 2.03 times more than that of a non-build process bug, and the number of source files modified to fix a build process bug is around 2.34 times more than that modified for a non-build bug.