The errors, in no particular order, found in Figure 2.6.15 are these:
1. All but one of the data flows have a single-character name; these should be replaced with clearly functional names.
2. Bubble 3 has no output. There is no reason for a process to have input but no output.
3. Bubble 4 has no input. Apart from a random number generator, or the production of pure fiction, processes must have some kind of data input to transform into their output.
4. The data flow SOMETHING TO DO WITH INVOICES is badly named. It appears ...