Two metaphorical descriptors of professional learning that keep me up at night...

Christmas tree effect

"Many leaders base their leadership behavior on the assumption that the most effective way to improve schools is by implementing hot new intitatives.  In district after district, school after school, they can be observed frantically implementing whatever is new, whatever the district next door is doing, one initiative after the other.  The goal is to do, do, and do more.  Rarely do they stick with anything long enough to see if it's getting results...Eventually these schools (and their districts) begin to resemble a Christmas tree -- covered with program ornaments of every new educational practice or fad, leaving faculty and staff feeling completely overwhelmed" (Eaker and Keating, 2012, pp. 8-9)

Shotgun professional development
In shotgun professional development, each day, week, month or year yields a new focus. Teachers who have experienced this type of PD say things like, "Last year, we focused on formative assessment, but this year we're focusing on positive behavior intervention supports" and are unable to form a connection between the two.   Teachers may also be asked to implement a new instructional strategy in the fall and then the strategy is never discussed again the rest of the year.  The faculty may see change as the "flavor of the month" rather than a systematic effort to improve school culture and/or school achievement, broadly defined.

To use a curriculum analogy, shotgun professional development is often a mile wide and an inch deep.  This type of professional development may lack follow-up and teachers may suffer from what Doug Reeves calls initiative fatigue.