hmmmmmm. that's going to be difficult to catch. I left it running for about 10 minutes and everything seemed to behave fine but like you say it just happens at some point. I'll add it to the bug list but I can't really fix in until I've got a method of testing it so if you come across one please let me know. Thanks.
It seems to be in the interactions with other bots (i.e. them standing in the way, I guess), as this does not happen when I put him into his own little area with nothing but the two required storages, the furnace and fences all around...
Will try to find a minimal setup where the problem occurs.
(Also, I can now confirm that bots that are unreachable for the recharge bots get charged remotely^^)
Okay, it seems to be definitely related to other bots blocking their way:
I've now got a setup where he skipped the step of actually walking to the furnace and jumped straight to trying to insert the item!
http://s000.tinyupload.com/index.php?file_id=72337184487671410134