A few days ago I visited a local garden centre and came across this plant, which goes by the name of Salvia "Hot Lips." A bee was doing the rounds of the flowers, but I noticed that it was not entering the flower through the corolla tube to collect the nectar. Instead it was inserting its tongue at the side of the tube, just above the sepals. On closer examination, I discovered that there was already a small hole in the side of the corolla tube at this point (which can just be made out in the picture above), and the bee was making use of this to gain access to the nectar. As it flew round the plants, it inspected each flower in search of a hole, and did not attempt to collect nectar from flowers without holes. What is more, the holes seemed to be only on the right side of the flowers when facing them, and not on the left.
Charles dawin noticed this behaviour by bees, and he and other writers give the most likely culprit for the initial piercing to be bumble bees, with honey bees subsequently utilising the holes.
Free, J. B. 1993. Insect Pollination of Crops. p.267-9
Maloof, Joan E; Inouye, David W. Ecology 2651-2661 81, no. 10 (Oct 2000): p. 2651-2661 Are nectar robbers cheaters or mutualists?
Charlton, Nicholas L. British Entomological and Natural History Society. When bees exploit plants: Nectar robbery