1. Not researching the fees
The fees or expense ratios of most ETFs are less than other traditional investments, but that doesn't mean there aren't variances within that space. There are indexed ETFs and actively managed ones, and the former typically have lower fees than the latter.
If you think expense ratios don't matter, consider this example: You have an ETF that you originally invest $5,000 in and add $1,000 each year (with an average return of 6% annually). If the fund had an expense ratio of 0.25%, you'd pay $5,357 in fees over the course of 30 years. But if the expense ratio had been 0.75%, that figure would jump to $15,205--a difference of almost $10K.