Note: Whether a glitch is normal or indicates a problem depends on many factors best evaluated by a professional, such as a geriatric psychiatrist or neuropsychologist. It's important to realize that changes in cognition and memory tend to fall along a spectrum. This symptom is considered a sign of concern that warrants an evaluation if it happens consistently or begins to interfere with daily life, especially if this is a change (new or different).
Why it happens
Problems with money and finances are among the earliest red flags of cognitive trouble. Researchers think this is probably because money management involves so many different skills, from making sound decisions to remembering basic math, from the details of adding a column of numbers to the broad implications of whom to trust.
What you can do
Don't write off money mistakes as "normal aging" or simple mistakes. Everyone makes mistakes, but when the frequency or magnitude of errors grows, you're ignoring them at your own risk.
There's no need to be paranoid, but you do need to be very careful.
Consider any concerns about managing money to be a wake-up call to arrange a durable power of attorney for finances. Whether or not the day comes when this is necessary, it might -- and it's far more prudent to prepare for the possibility while your loved one still can make informed judgments.
Beware of scams and other kinds of abuse relating to money management. Familiarize yourself with the most common scams and frauds.
Take a more active role in joint financial planning and bill paying; don't leave things up to the person making mistakes, even if this person has managed independently in the past.
Automate payments for as many regular household bills as possible, so none are neglected.
Consider suggesting that he or she no longer carry large bills or unnecessary credit cards (or a debit card). Obviously we all need money to function, but it can be wise to safeguard against unnecessary disasters.