My plumber installed my hot water baseboard backward. The feed and return are hooked up backward. What problems will this cause?
Assuming it's a hot-water baseboard heater, there should be no problem from which way the water goes. It's a rather simple machine (pipe with heat dissipators) so it couldn't care less which direction the hot water is flowing.

What could be an issue is if a bleeder valve was installed on the output side and not the intake side. Typically the intake side would collect more air to bleed.