Multiple ports that depend on graphics/openjpeg cannot be built if both graphics/openjpeg and graphics/openjpeg15 are installed.
The header openjpeg.h is installed to /usr/local/include/openjpeg-2.3 by graphics/openjpeg, but the header with the same name is installed to /usr/local/include by graphics/openjpeg15. When a port is building, both /usr/local/include/openjpeg-2.3 and /usr/local/include are configured as include directories, so the port dependent on graphics/openjpeg tries to include /usr/local/include/openjpeg.h and then fails to build.
Example of such port: graphics/simpleviewer.
Moreover, if a port needs devel/doxygen and graphics/openjpeg in the same time - it cannot be built at all as devel/doxygen (indirectly) depends on graphics/openjpeg15, so graphics/openjpeg15 will be installed before the port and breaks the build.
The port graphics/openjpeg15 must install the header openjpeg.h into /usr/local/include/openjpeg-1.5 and change his pc file accordingly to avoid such conflicts.