Never attribute to malice that which is adequately explained by stupidity—but don't rule out malice.
The addition matters. Hanlon's razor can become a way of never looking at motive, and Heinlein's version keeps the possibility open without making it the default.
In practice
- A supplier whose invoices are always wrong may be incompetent, and the pattern of errors always favouring them is worth noticing.
- A colleague who repeatedly forgets to copy you in may be disorganised, and the meetings they forget are consistently the ones where your view would be inconvenient.