Not saving for retirement.
Many millennials are early on in their careers, making less money than is desired and paying off more loans than graduates in the past. So retirement is often the last thing on their minds. What they fail to comprehend, however, is that when it comes to retirement funds it pays to have a lot of time. The earlier they start saving, the more money they will ultimately have when it comes time to retire.

Not saving at all.
Millennials often think that since they're not making much money right now, they can't really grow their savings. But even small savings still count, and it's good to get in the habit of setting money aside each pay period.

Not investing enough.
Investing has always been a foreign concept to 20-somethings, and they're weary to spend money on something they don't completely understand. Which makes sense, in light of the major market meltdown witnessed by millennials as they were entering the workforce. However, they need to remember that the market will ebb and flow, but ultimately has an upward trend in the long-term.

Living beyond their means.
Being new to their careers and the benefits associated with those careers, it's easy for millennials to get carried away and live beyond their means. Low cost subscription services like Netflix and Spotify seem like chump change, but it's easy to overlook how expensive services like these are in the aggregate. Saving just a small percentage of their earnings will allow millennials to easily live within their means.

Neglecting tax-advantaged opportunities.
When millennials start their new careers, they often join a new income bracket. But most are probably unaware of the tax benefits available to them based on their new income status. They should really work to understand how taxes work. It might also be a good idea to consult a tax professional to ensure they're doing everything they can to maximize their tax benefits.