The bed project I am currently working uses a lot of tenon and mortise joinery. I have to admit I do get my tenons and mortises wrong sometimes.
When the piece is an 8ft long section of wide oak, it's hard to start over. The tenon needs to be fixed. This post should get you started on how to fix your mistakes.
1) The tenon won't fit into the mortise
Wood needs to be removed. The best tool for the job is a shoulder plane. It was practically made for the task. If you don't have one sand paper will do.
Remove material from both sides of the mortise or tenon. If you only remove material from one side, alignment to the pieces that are joined changes. The tenon will be off center. Measure carefully. Make sure the reveal is still equal around all sides.
2) Tenon is too thin or the mortise is too wide
If there is too little wood, we need to add more. In most cases it should be possible to glue a thin strip of wood onto the tenon. Once the glue dries, the tenon will probably be too large for the mortise. Plane it too size as you would do with an oversize tenon.
3) The edge of the reveal is not even
This problem is harder to fix. You can use sand paper or a shoulder plane to create a straight edge all around the tenon. This will remove material. The tenon will get longer. The way the piece fits changes too. If that's a problem, you have to built up the edge first. Attach a piece of wood with glue, try wood putty.