Wearing crocs on Tisha B'Av
מעיקר הדין we pasken that only leather shoes are prohibited. However, nowadays, we have shoes that are not made of leather which are as comfortable or more comfortable then leather shoes. Crocs are a good example, I see people wearing them everywhere. Give this, it may very well be that they are prohibited on Tisha B'Av. Leather is not a הלכה למשה מסיני. Leather is prohibited because that is what shoes used to be made of and people did not walk around with non-leather shoes. Today, where people do wear non-leather shoes all over, the din may change because the facts changed.
In any case, there is no question that wearing the same shoes that you wear all week on Tisha B'Av certainly violates the spirit of the din. Not wearing leather shoes is supposed to be an עינוי, you are supposed to feel uncomfortable. If you wear super comfortable non-leather shoes you have defeated the whole purpose of the din. By Aveilus there is supposed to be a kiyum balev, you are supposed to feel something. If you wear your regular shoes there is certainly no kiyum balev.