Regrettably, especially given the selfless and courageous efforts of those running the escape lines, they and evading aircrew were also prey to collaborators infiltrating those lines. Largely the work of one man, Jacques Desoubrie, 168 Allied aircrew shot down over France in 1944 were betrayed to the Gestapo. They did not go into the normal Luftwaffe-run POW system, but were held instead in Fresnes Prison near Paris, in close confinement and in poor conditions. In mid-August 1944, as the Allied advance neared Paris, the prison was emptied, the men were packed into boxcars for a 5-day rail journey to Buchenwald where conditions were immeasurably worse. The Luftwaffe eventually became aware of this illegal imprisonment and secured the release of the 166 surviving men to Stalag Luft 3 in late October.
I have to admit to only having become aware of the fate of this group of airmen a few years ago, and I understand that there was a degree of official unwillingness to publicise their treatment on return. A book, "168 jump into Hell" by Arthur Kinnis and Stanley Booker was published in Canada in 1999, and relates the whole story.