As of the Ruby mongo-1.1.4 gem, GridIO#read_all is defined as:

1. Read a file in its entirety.
   def read_all
   buf = ''
   while true
   buf << @current_chunk['data'].to_s
   @current_chunk = get_chunk(@current_chunk['n'] + 1)
   break unless @current_chunk
   end
   buf
   end

The shortcut of setting @current_chunk = get_chunk(next_n) has a side effect of breaking anything that depends on @current_chunk. read_length handles this properly, though it may be a few more instructions than is required when doing a read_all.

WORKAROUND:
Using GridIO#read_length(file_length_or_remaining_length) doesn't break further seeks, etc.