Here's a strange example!
Colin Wright of the Shropshire Ornithological Society reported that, 'early season', a pair of robins had used a nestbox designed for blackbirds. No sooner had the robins nested than the blackbirds came along and evicted them, taking over the nest and laying their own eggs.When the chicks hatched though, they were a mixture of different species, some robins' eggs having stayed put. Whilst the blackbird parents ignored them, the robins had astonishingly hung around and managed to successfully rear their young with feeding sorties when the nest was unguarded. [Reported in The Countryman for December 2008 ]