Top sunk in middle: Too much raising agent.  Over-creaming of fat and sugar.  Over-beating after egg added.  Mixture too wet.  Tin too small.  Under baking.  Oven too cool.  Slamming oven door during baking.

Over-browning of top, hard crust: Oven too hot.  Over-baking.   Cake too near top of oven.  Tin too large.  Too much sugar.  Fruit cake taking several hours baking not protected with paper.

Insufficient browning: Insufficient sugar or egg.  Mixture too wet.  Under-baking. Oven over-loaded.

Cakes sticking to tin or sides and bottom too brown: Poor quality tin.  Tin insufficiently greased and/or lined.

Cakes forms crusty ring round sides:  Tin over-greased.

Speckling on top:  Raising agent and flour poorly sifted.  Too much sugar.  Granulated sugar used.  Insufficient creaming.

Tunnelling in centre of cake and uneven texture: Over-mixing or uneven mixing when adding flour or liquid.   Mixture too dry, causing air pockets.  Raising agent and flour poorly blended.

Fruit sunk:  Mixture too wet.  Fruit wet.  Glace fruit syrupy (wash, dry and flour well to correct this).  Fruit too large and heavy for mix.  Oven too cool.  Opening oven too soon.  Too much raising agent.

Coarse texture: Fat not rubbed in or creamed properly.  Inadequate mixing.  Too much raising agent.  Oven too cool. 

Rubbery texture:  Over-mixing.  Too much egg and/or milk.

Dry, crumbly texture and stales rapidly:  Mixture too dry.  Fat not rubbed-in or creamed sufficiently.  Too much raising agent.  Baking too slowly. 
Close texture:  Too little raising agent.  Too much fat, egg or flour.  Mixture too dry or too wet.  Over-mixing.  Inadequate creaming and/or beating.  Oven too hot.  Under-baking.

Uneven rise:  Oven incorrectly preheated.  Oven shelf or oven not level.  Cake not in centre of oven/shelf.

Small cakes spread:  Mixture too wet.  Not enough fat.  Too much or too little raising agent.  Too much mixture in cases.  Oven too cool.