I'd say most common mistakes are:
-Not cleaning properly/adequately
-Not test aligning EVERYTHING before final placement. If you don't you will almost always end up with one end too short, text ending up in an opening or door handle, or everything being crooked.
-Cutting/trimming with a knife directly on the paint. DON'T DO IT. Underlay teflon tape or use a knifeless tape.
-Not post-heating, especially tight curves and edges.