I made a cake for my brother's birthday today and when we sliced the cake the filling layers were riddled with crumbs from the previous layer when it was being cut through.
I used box mix and chocolate bettercreme. When I leveled the cake I removed all the crumbs and was careful when putting the filling, to not have crumbs in it. But when the cake was sliced it didn't have the clean slices.

Can someone please help me as to what went wrong? Was the cake dry and overcooked? I took it out of the oven before I usually do also.

Sometimes it is the knife that is problem, not the cake. If you aren't using a clean sharp knife, and wiping it after every cut, the gunk on the knife will make your cake start crumbling.

I never even thought about that. We were at a restaurant and they gave us a cake knife server, which wasn't sharp at all.

sharp knife essential, i am a bit ocd about people cutting my cakes with blunt knives !!!!!!!!!!!!

That is something I definitely have to consider now, when someone ask me to make them a cake. Presentation is key and when it doesn't look clean when you slice, it won't taste as good because we eat with our eyes.