The International STa"M Forum

This year I am doing a siyum on Chol hamoed. It's the fifth time I have done a siyum on Chol Hamoed . If the outline of the remaining letters is written before Yom Tov and the siyum involves just filling in letters,it is permitted to do so on chol hamoed because such work is not considered maaseh uman. It is for this reason that it is not uncommon for a siyum to occur on chol hamoed. My question is, if something goes wrong i.e. there is a smudge or mistake that needs to be repaired during the siyum process - and the skill needed to fix it is masseh uman  - ie we are not just filling in letters anymore, this is a proper tikkun - what is the din? Thankfully it has never happened to me, but I was wondering what happens in such a case and if anyone has experience in this matter. A gut Moed!

B"H I understand that in general there is an allowance to check a Mezuzah that was already given to check before the chag (and fix even Maseh Uman) if it is needed for the Chag, for this is considered Daver Haaved and Daver Mitzvah (see Shmiras Shabbas Kehilchasa 67:38). I would like to clarify if this can work even if it is a doubt that the recipient will receive their Mezuzahs during the Chag. On the other hand, if they will be checked only after the Chag they will have to wait even longer for them to place up their Mezuza(s). I would appreciate your comments.