The Bug:
Sometimes, a simple command like /tp @p 0 50 0 may not work, because '50 is not a number'

This actually seems to be caused by an invisible character in the command, and this can be proven by deleting the text around the invalid part of the command and seeing if it takes 1 more press of the delete key then there are visible characters that you deleted.

How to Recreate:
As it turns out, this bug is very easy to recreate! Just press the up or down arrow while writing a command in a command block and you have just typed the invisible character!