Why did sparks fly when testing outlet frequency?
If you drew sparks with only a single probe, it suggests that you accidentally shorted across neutral to hot somewhere on the outlet.

Studying and understanding the manual and the limits of the meter would likely help you to avoid such mishaps. (Or even worse such as fire or electrocution.) Any kind of measurement of mains power should use considerable caution.

Unless the meter is rated for measuring mains frequency, it would be better to avoid measuring high voltages like that. For practical purposes, the frequency of the utility power is the least likely to change or be worthy of measurement. Unless you were getting power from a portable generator or some "off-grid" source.