 Does this look familiar? It's what appears on your oscilloscope when your probe is left ungrounded, though it may look a bit different depending on where you live. That spiky AC component has a frequency of 60 Hz, the same frequency used by main supply voltage here in the US. Yep, this 140 millivolt peak-to-peak signal is being generated by my wall power here. Without a reference to circuit ground, the scope's probe will pick up noise generated by nearby power sources. So consider seeing this signal a reminder to please connect your ground clip. Much better.