A DESCRIPTION OF THE PROBLEM :
For many engineers I suspect Stream Gatherers will be the first time they're exposed to streams in detail and it occurred to me that it's not immediately obvious that a push is an inversion of usual stream operation and requires all pushed elements for a call to integrate to be buffered.

An @implSpec similar to those on the Stream methods like distinct/mapMulti that buffer could help avoid one-to-many cases in particular from catching folks out.