cellio | daf bit: Yoma 34

The talmud describes in detail the rituals performed by the high priest on Yom Kippur. Several times during the day he washes his hands and feet and sometimes he immerses fully. The mishna teaches that if he is old or of delicate health they heat this water for him. The g'mara elaborates: R. Yehudah said that lumps of wrought iron were heated in the afternoon before Yom Kippur and then put in the water to warm it up. But, some protested, doesn't this harden the iron? Abaye said it didn't reach the point of hardening, but even if it did, a forbidden act (hardening) that is produced without intent is permitted. (We learned this in tractate Shabbat. Unintended side-effects are ok.) But wait, R. Yehudah elsewhere said that side-effects remain forbidden even if unintended -- how then can he say this is ok?? The resolution: when R. Yehudah said that side-effects remain forbidden he was talking about direct torah commandments (d'oraita), but we are more lenient with rabbinic commands (d'rabbanan). Since the restriction on hardening is rabbinic, and since hardening the iron is an unintended side-effect of heating the water, there is no problem. (34b)

(As with all of my posts, please remember that I am not a rabbi and this is not p'sak, a legal judgement, and if unintended consequences of work on Shabbat or Yom Tov are a practical matter for you, you should consult your rabbi. Especially since the talmud doesn't necessarily have the last word on anything...)