In synaptic vesicles, some neurochemists have suggested that vesicles occasionally may not completely fuse with presynaptic membranes in neurotransmitter release into the synaptic cleft.  The controversy lies in whether or not endocytosis always occurs in vesicle reforming after release of the neurotransmitter.  Another proposed mechanism for release of vesicle contents into extracellular fluid is called kiss-and-run fusion.
There is some indication that vesicles may only form a small pore in the presynaptic membrane allowing contents to be released by standard diffusion for a short while before retreating back into the presynaptic cell.  This mechanism may be a way around clathrin-mediated endocytosis.  It is also proposed that the vesicle does not need to return to an endosome to refill, though it is not thoroughly understood by which mechanism it would refill. This does not exclude full vesicle fusion, but only states that both mechanisms may operate in synaptic clefts.
"Kiss and run" has been shown to occur in endocrine cells, though it has not been directly witnessed in synaptic gaps.