If you supply insufficient data in the dataId for a butler to put() the data, it will just pretend to have done it (i.e. it doesn't raise or warn, it just silently returns as if all was fine).

If one does

butler.put(img, 'postISRCCD', dataId={'visit':123,'filter':'r'}) 

where the postISRCCD template looks like
template: "postISRCCD/postISRCCD_v%(visit)d_f%(filter)s.fits"
then this will work. However, if you provide an empty dataId dict, it will not complain, but nothing will get written (of course).

This seems like a moderately serious problem, as it is easy to accidentally under-specify or mis-specify a dataId.

(FWIW, if I change 'postISRCCD' to an undefined dataset type I do get an error).