Jim Yanik wrote in
:

Tektronix used to offer a pamphlet about scope measurements and ground
safety....It explained why various
methods of "floating" a scope are unsafe.

IIRC,TEK now has something similar on their website.

Yep, here it is:

http://www.tek.com/Measurement/cgi-bin/framed.pl?
Document=/Measurement/App_Notes/Technical_Briefs/tds3000-
float/eng/limitations.html&FrameSet=oscilloscopes

Quoting from the web page,

"Floating" a ground referenced oscilloscope is the technique of defeating
the oscilloscope's protective grounding system - disconnecting "signal
common" from earth, either by defeating the grounding system or using an
isolation transformer. This allows accessible parts of the instrument
such as chassis, cabinet, and connectors to assume the potential of the
probe ground lead connection point. This is dangerous, not only from the
standpoint of elevated voltages present on the oscilloscope (a shock
hazard to the operator), but also due to cumulative stresses on the
oscilloscope's power transformer insulation. This stress may not cause
immediate failure, but may lead to future dangerous failures (a shock and
fire hazard), even after returning the oscilloscope to properly grounded
operation!

Not only is floating a ground-referenced oscilloscope dangerous, but the
measurements are often inaccurate. This results from the total
capacitance of the oscilloscope chassis being directly connected to the
circuit under test at the point where the common lead is connected.

WARNING

Never attempt to defeat the protective grounding system of your
oscilloscope by using an isolation transformer or disconnecting the
ground connector on the power plug. Failure to follow safety warnings can
result in serious injury or loss of life.