By Rabbi Berach Steinfeld  
                In Gemara Brachos 8b and Menachos 99b we learn from the fact the broken pieces of the first luchos were kept inside the Aron in the mishkan that we have to be careful with a zaken who forgot his learning because of an illness. Rashi clarifies that this refers to a talmid chacham who forgot his learning via illness or because he did not have parnassa and had to go to work.
The Shela in Meseches Shavuos, Perek Torah Ohr 12 writes that based on the gemara in Eiruvin 54a, had the luchos not been broken we would have had the ability to learn Torah and never forget our learning. This is the pshat why one should be careful and show respect to somebody who forgot his learning; it is not his fault he forgot his learning since after the luchos were broken Klal Yisroel started to forget Torah.  
In the event one causes himself to forget learning willingly, i.e. he does not learn or review despite having the capability of doing so, one is not required to show him respect. The gemara in Yuma 38b says we remove him from his leadership post.
What kind of respect must be shown to a zaken who forgot his learning via illness or other extenuating circumstances? Do we need to show him the same respect as he originally had, or is the requirement not to demean him? The Kehillas Yaakov in his Igros writes that the halacha is not clear. He writes this based on the gemara in Menachos 99a that says one should not be noheg a minhag bizayon. It is self understood that one is not allowed to embarrass him; since it is not his fault he forgot his learning. On the contrary, you must show respect, we are just not sure to what extent.
The Yerushalmi Moed Koton 3:1 says that a talmid chacham that forgot his learning should be treated like the kedusha of the Aron. The Korban HaEida explains that you treat him with the same respect that Moshe afforded the broken luchos that were put in the same Aron as the new luchos. The Korban HaEida quotes a Meiri in Brachos 8b that says that a talmid chacham that forgot his learning must be honored as if he never forgot his learning.
The Shaalos U’Tshuvos Imrei Yosher in Vol 3 Pesachim 68b asks the following question. The gemara in Pesachim 8b says that on every Shavuos Rav Yosef would serve a strong and tasty calf to celebrate Matan Torah. He would say if not for that day there would be plenty of Yosefs in the street, but now that there was Matan Torah he stands out by being a talmid chacham. The Be’er Hetev in Orach Chaim 494 asks, we know that Rav Yosef was an anav as we see in Sotah 49b that Rav Yosef says that anava still existed because he was still around. How was Reb Yosef exalting himself regarding the Torah he learned? The Imrei Yosher answers based on Gemara Nedarim 41 that Reb Yosef became ill and forgot his learning, so at first glance it may seem that one would not need to show him respect. But since we find that the broken luchos were put in the Aron, we see how important it is to show respect to the talmid chacham who forgot his learning. Now we can understand what Reb Yosef was saying when he said if not for Mattan Torah I would be like anyone else; but now that we see that the broken luchos were in the aron, he celebrated the fact that despite having forgotten his learning via illness he still was honored due to the Torah he had known.
May we be zocheh to remember our Torah.
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