ציצית - wool or cotton?
The daf yomi just learned a sugya that discussed what the Torah calls a בגד in various areas of halacha, tumas negaim, tumas sheratzim, and ציצית. It comes out according to some of the opinions in Tannaim that only צמר ופשתים are considered a בגד for ציצית.
There is a machlokes how to pasken. The Rambam and in his footsteps the שו"ע paskens that only wool is חייב in ציצית min hatorah, and the Rama paskens that all types of materials are חייב in ציצית min hatorah. The mishna berura suggests that a person should be machmir and wear wool.
The Gra and the Chazon Ish both didn't wear wool because they held that we pasken like the Rama and they wanted to emphasize that. We see from here the tremendous differnce in approach in psak between them and the Mishna Berura (as Dr. Chaim Soloveitchik pointed out in his article Rupture and Reconstruction). The Mishna Berura sees 2 opinions by ציצית so he says you should try to be יוצא both. The Chazon Ish and the Gra on the other hand held that we pasken like the Rama and therefore we should do like the Rama.
In today's world the Mishna Berura's method is the prevalent one, and that is one of the reasons why we have so many chumras today.