as it says. on my shaman, i have saved a number of normals from wiping by dropping earth elemental totem when the tank went down as elemental, by CC-ing a loose mob, or even by healing for a short while when the healer went down. this was in level 85 normals.
on my druid, i have stopped countless wipes by switching to bear form when the tank died (all pre 60 instances). i have also sometimes managed to heal and tank at the same time, by loading up HoT's, switching to bear, tanking until the hots wore off, rehot myself, load hots up again. a wailing caverns misspull comes to mind.
on my shadowpriest, i often heal a group once the healer leaves unexpectedly.
anyone else who wants to share his experiences?