Missing the 3rd of a chord – unless the chord has a 2 or 4 in the figure, a 3rd is almost always the most important note to add, so the harmony is complete and the quality of the chord is clear
When a 9-8 suspension is present, the 3rd is required unless you are writing a two-part texture.
Not preparing suspensions – if you plan to write a suspension, prepare it with a tied consonance in the prior beat of the same duration
Incomplete 6/3 chords – in 3 or more parts, 6/3 chords must be complete
Incomplete 5/3 chords for no good reason – we are looking for a rich texture – so dropping notes for no reason doesn’t make much sense
Incomplete suspended sonorities – 4-3 implies an added 5th, 7-6 an added 3rd.
Parallel octaves or unisons (especially several in a row) – this is a sign that you are writing the same voice twice
Parallel fifths – also not good in this style
Using diminution without splitting into voices or parts – diminution divides notes in one part – if your notation doesn’t make it clear which part is being divided the result is ambiguous and you will make voice leading errors.
Using diminution in multiple parts without regard to parallels and resulting harmonies