Sweetlips aren't reef safe. And they get huge. 86 cm according to Fishbase.org - that's about 2 1/2 feet. They are also tricky to keep - juveniles often won't eat. Once they're eating they usually do OK but I wouldn't put one in a reef - and you'd definitely need an exit plan for it.
arrow crabs can be problematic. they eat everything that is small and wiggles. so if you like feather dusters than do not get an arrow crab. they can also be aggressive towards other crabs in the system.