1) Using someone else's tileset all times
2) Using someone else's music in large levelpacks
3) Never making a 10x7 (or 20x15) picture for layer 8 in tilesets
4) Always using 640x480 instead of 320x200
5) Forgetting one or more of the background layers
6) Using layer 5 as sprite background (like Swamps by Agama)
7) Using a textured background WHILE using layer 5 as sprite background (same as above)
8) Making a background of composable parts instead of a whole looping picture (like the households in Megatropolis and Palmtree Plaza)
9) Using a drawing style too much different from the sprites style (like Blade's crayon style or Disguise's texture made parts)
10) Leaving Jazz too much room to jump and run all over the map
11) Forgetting to apply eyecandy to tilesets or to use it in levels
12) Making levels larger or smaller than 16384 tiles (256x64 or 128x128)
13) Using musics longer than 2:30 (like DreamsOfHope2) or shorter than 1:30 (like alarm or tralala bim boum)
14) Using musics that don't have a looping part, so that they fade and restart (like AWS_FIRE or Mayhem)
15) Making tileset night version from halfing luminance instead of completely changing the colors (like ET's fungi swamps)
16) Being too lazy to make new levels (VERY guilty)
17) Using tiles from other videogames (like Kejero with Donkey Kong)