Interesting app note from Maxim Integrated on small parasitic effects of capacitors that makes them less passive. Link here (PDF)
Active components like transistors and integrated circuits change signals using energy from the power supply. Conversely, passive components like resistors, capacitors, inductors, and connectors do not consume power or so we like to assume. However, passive components actually can and do change the signal in unexpected ways because they all contain parasitic components.
This application note, the first in a 3-part series, discusses parasitic capacitance.