If you're buying something with low liquidity (e.g. an ETF with low AUM or a small-cap stock) the bid/ask spread can be huge. When I first started investing/speculating I made the mistake of putting in market orders without checking the spread. That resulted in me paying $1 or more per share above the last quoted market price. E.g. the last market quote was $65, but the going ask was $66. After-hours market orders (even on large-cap stocks or high AUM ETFs) can also cause some nasty surprises.
