It's transient, so might not show up easily. What I see is that occasionally a rim load will stop on the first binary word, not even loading the bootloader. It seems to only happen if I've loaded tapes quite a few times in the same session. Remounting the tape and trying again then works.
I should note I'm using the web interface, so it might be specific to that. I don't think it's related to any of my changes, I haven't modified any of the code related to the ptr or the readin logic and I haven't touched the web intf code at all.
Bill