It is very common that we have two versions of dll: somedll.dll(for Release build) and somedll_d.dll(for Debug build). Because of the dll dependancy complications, if these two versions were load at the same time, it may not cause build errors. However, something wrong can happen during the run time.
For example, if there is a global variable in the source file whose output is somedll.dll/somedll_d.dll, that global variable may be initialized twice (once during the load time of somedll.dll and once during the load time of somedll_d.dll). The client code then will get confused on which of the two references to use. But the fact is, it should be only one instance of that global variable existing.