(at around 1 min) When Dirk Gently leaves Macduff in the pub, he also leaves his pint glass 3/4 full. MacDuff picks up his own glass and is still drinking when in the following shot the barman approaches - but Dirk's glass is now empty.
At 10:26 when the unpaid receptionist is in a hurry to leave she packs her framed picture twice.
At around the 28 minute mark when Dirk is having tea with Mrs Jordan and she put medication into her tea, he remarks that he hopes she is okay at least for long enough to pay the invoice. However he told her earlier that the invoice is payable in advance, and left with a wad of cash.
The paw prints in the warehouse are wrong. Like most four legged animals, cats step on their fore paw prints with their back paws.