The dāna given in improper place and time to undeserving people, without due respect and attention, and with contempt, is considered to be tāmasa.
In tāmasa dāna, the degeneration reaches its acme. The giver cares for no discipline, restraint or value in what he does. His moods and whims take over everything else. The giver is averse to considerations like good time, favourable place and deserving recipient. Often he selects the undeserving, a wrong time or place, inviting contempt and ridicule from the gentry.
What makes dāna sacred is its relevance, usefulness and benevolence. When the whole performance is subjected to inattention and neglect, it loses all nobility and sublimity.
How does the human mind host such utterly contrasting notes, making life a constant challenge for one and all? By delineating the effects of all gunas, Krishna exhorts everyone to reduce rajas and tamas, and enhance sattva guna. This done, life becomes most desirable and a blessing to others as well.