Salaam, Here is an example; a tasbih (a string with beads, dividing beads, and counters) is a bid'ah. It was not used by Rasulullah alaihi wa salim, and there are hadith that tell us he used to count azkaar on his fingers. Some ulama have said that tasbih should not be used (Shaikhul Hadith Muhammad Zakariyya mentions this in Fadaail Amaal) because of this. Many ulama and salaf (including our Imam Hasan Basri) have used tasbih and have praised their use. Tasbih is a means to an end; its only value in deen is that it facilitates ease of zikr, but IF it is thought to be NEEDED to make zikr, then it would become an evil innovation. It has no deeni value in and of itself, the only value it has is that it is a means to a deeni end, ie remembrance of Allah. But if the intention was changed and people started to think (as some people do) that a tasbih is a must, and they will make tasbih, say Allah's Name, tahmid, etc, when they have a tasbih, but if they don't they won't, this is a problem. So a tasbih is a bid'ah, that can be merely permissable, to praiseworthy, to deviation depending on the person's understanding. Also you could take the pas anfas and habs dam methods of zikr ( which is wide spread among Deobandis and others); the Prophet salallahu alaihi wa salim did not mention these types or methods of breathing control, holding the breathe, holding the breathe under water while making zikr, etc, but what we have been ordered to do is to remember Allah at all times. Now these methods which facilitate zikr are not from the Prophet salallahu alaihi wa salim, BUT they have a value because they meet a deeni end. They could however become evil innovations if people started to think that these methods are ibadah, or that one must do these methods in order to remember Allah all the time. Same with mawlid; did the Prophet salallahu alaihi wa salim say to celebrate it? Not that I am aware of. Did the Sahabah celebrate it? Not that I am aware of. But I have also never heard of the Sahabah doing pas anfas, habs dam, or using tasbih (although with all of these things there are indications in the sunnah). Can mawlid become an evil innovation? Yes, if one thinks that the only way to gain the love of Rasulullah salallahu alaihi wa salim is by celebrating the mawlid, or if people think it is a birthday party for one day, or that it is ibadah. All the things that would make the others things I mentioned evil innovations would make the mawlid an evil innovation. The mawlid is a method found useful by ulama to increase love for the Prophet salallahu alaihi wa salim, it is not nessecary, but it is useful and many people have come closer to deen because of it. If we are going to totally reject the mawlid, we should also reject the use of tasbih, pas anfas, habs dam, and the many many other things that the Prophet salallahu alaihi wa salim and his ashaab did not do or institute.