DMM Input Capacitance

[Kleinstein]

The measurement frequency can make quite a difference, as the input can have some resistance (and maybe inductance) from the protection in front of the main input capacitance. With a high test frequency the resistance can hide the capacitance, especially if large. With a low frequency series resistance is less relevant.

For the use of the DMM the additional resistance can also make a difference: it can be enough that loading an OP-amp is no longer an issue.

[KungFuJosh]

In practice, how often do you drop clipped wire ends and stuff into the mains and USB outlets? 

Never. The outlet is over a foot from my heat resistant work pad. However, if I start doing that now, I'm blaming you. 😉