Ok, so after the D'uras sisters release Geordi back to the Enterprise, Dr. Crusher fixes his visor and plants it back on his head. And thus, two mistakes are born. First, as Geordi has indicated on various accounts, he is not able to see like normal humans (he sees in a spectrum, including heat). However, the D'uras sisters have no problem viewing the content that his visors transmit back. And then, when he enters engineering and looks at the shield frequency, it is at a stable number. Every good Trek fan knows that shield frequency is constantly and randomly modulated because of the encounter with the Borg!