The wooden bombs have been discussed earlier on this forum, see http://forum.axishistory.com/viewtopic.php?f=49&t=121451
. The whole story is very unlikely and has so far never been supported by facts or original documents.
The source of it may have been German (concrete) practice bombs being dropped on a training target on or near a (dummy) airfield and the German plane being mistaken for an allied air attack. Some Scheinflugplätze did indeed have a double function as bombing practice range too.
Or perhaps real allied bombs being dropped but not heard or seen to explode. Keep in mind that during the earlier years of the war RAF bombing missions were usually on a very small scale, with often just one or two Blenheims or Hampdens attacking a target with a few relatively light bombs.
And as we all know.... tell one story and it goes around tenfold!
Rgds, René D.