Let's say someone has planned to be away from home during Pesach but finds out that his trip will be ending early and he will be home and needs to cook in his house. Can he kasher his utensils during Hol Hamo'ed?
The Rama writes (OC 452:1), after the Shulchan Arukh cautioned to Kasher everything before the fifth hour on Erev Pesach morning to avoid various complications:
ואם לא הגעיל קודם זמן איסורו יכול להגעיל עד הפסח שאז חמץ במשהו ואינו מועיל הגעלה שחוזר ובולע אבל מותר ללבן כלי תוך הפסח
And if you didn't Kasher before it became forbidden you can still Kasher until Pesach starts, for then Chametz [prohibits] even in minute amounts and boiling won't work for [the vessel] returns and absorbs [the minute Chametz flavor from the water], but it is permitted to Kasher a vessel via fire even during Pesach.
The Rama here is consistent with his earlier position (447:10) that Chametz flavor prohibits even when it imparts a negative flavor, but the Shulchan Arukh there thinks that such flavor does not prohibit. Accordingly, it might be permitted according to the Shulchan Arukh to Kasher via boiling water on Pesach if you were unable to Kasher before Pesach and the vessel hasn't been used in 24 hours so that its absorbed flavor has gone rancid.