Probably due to the thumb size < original size rule. Those should give HTTP 412 or something, and then the job can return "true" in run() for such cases, rather than logging and retrying two more times (and logging).
2015-07-23 08:31:09 mw1010 commonswiki runJobs ERROR: ThumbnailRender File:Silene_otites_002.JPG transformParams={"width":"640"} (uuid=fbfbc971ef4745f39f4c1dae81c87570,timestamp=1437640260,QueuePartition=rdb1) t=8620 error=ThumbnailRenderJob::run: incorrect HTTP status 500
2015-07-23 08:32:08 mw1004 commonswiki runJobs ERROR: ThumbnailRender File:Capitaine_de_vaisseau-IMG_9252.JPG transformParams={"width":"1280"} (uuid=e10f0d79ba4e4e6d820d0ccc4a162c66,timestamp=1437640319,QueuePartition=rdb1) t=8224 error=ThumbnailRenderJob::run: incorrect HTTP status 500
2015-07-23 08:32:08 mw1007 commonswiki runJobs ERROR: ThumbnailRender File:Capitaine_de_vaisseau-IMG_9252.JPG transformParams={"width":"800"} (uuid=b01d5e67754b40c2bca5149b4ff10470,timestamp=1437640319,QueuePartition=rdb1) t=8227 error=ThumbnailRenderJob::run: incorrect HTTP status 500
2015-07-23 08:32:08 mw1004 commonswiki runJobs ERROR: ThumbnailRender File:Capitaine_de_vaisseau-IMG_9252.JPG transformParams={"width":"1024"} (uuid=cdd28d7220054f02a9bd1138f65865ab,timestamp=1437640319,QueuePartition=rdb1) t=8257 error=ThumbnailRenderJob::run: incorrect HTTP status 500
2015-07-23 08:32:08 mw1002 commonswiki runJobs ERROR: ThumbnailRender File:Capitaine_de_vaisseau-IMG_9252.JPG transformParams={"width":"320"} (uuid=7f0ca2623ed24392ad8ca7c1ee8b1a96,timestamp=1437640319,QueuePartition=rdb1) t=8184 error=ThumbnailRenderJob::run: incorrect HTTP status 500
2015-07-23 08:32:08 mw1002 commonswiki runJobs ERROR: ThumbnailRender File:Capitaine_de_vaisseau-IMG_9252.JPG transformParams={"width":"640"} (uuid=5ccea45bd1254be0a30d2c2072a07d9f,timestamp=1437640319,QueuePartition=rdb1) t=8199 error=ThumbnailRenderJob::run: incorrect HTTP status 500
2015-07-23 08:32:42 mw1002 commonswiki runJobs ERROR: ThumbnailRender File:Silene_otites_003.JPG transformParams={"width":"800"} (uuid=eeec2978013c4ce588e53ab76a8eb425,timestamp=1437640353,QueuePartition=rdb2) t=8156 error=ThumbnailRenderJob::run: incorrect HTTP status 500
2015-07-23 08:32:42 mw1002 commonswiki runJobs ERROR: ThumbnailRender File:Silene_otites_003.JPG transformParams={"width":"640"} (uuid=a9975fc34238417eaf9620758c433f0f,timestamp=1437640353,QueuePartition=rdb2) t=8203 error=ThumbnailRenderJob::run: incorrect HTTP status 500
2015-07-23 08:32:42 mw1001 commonswiki runJobs ERROR: ThumbnailRender File:Silene_otites_003.JPG transformParams={"width":"320"} (uuid=36dceb3042354b7aa0605367e9092190,timestamp=1437640353,QueuePartition=rdb2) t=8232 error=ThumbnailRenderJob::run: incorrect HTTP status 500
2015-07-23 08:33:18 mw1016 commonswiki runJobs ERROR: ThumbnailRender File:Contre-amiral-IMG_9154.JPG transformParams={"width":"2560"} (uuid=0e65942249454dadae5888e6cfb9dd38,timestamp=1437640389,QueuePartition=rdb1) t=8219 error=ThumbnailRenderJob::run: incorrect HTTP status 500