Investment mistakes I see at every income level:
Low income earners - Reliance on Instant loans/Fuliza
Middle income - Accumulating plots of land that they are unable to sell/liquidate
High income earners - Using a lot of debt to get into real estate. Asset Rich, Cash Poor

Others that cut across all levels:
1. Saving money in a banks' savings or fixed deposit account with low returns.
2. Investing in things you don't understand
What else can you add?