Most writing about swing trading mistakes is about direction: you were bullish, the market wasn’t, you held too long. Those mistakes are real, but they are also the ones you can’t fully control. This post is about the other kind: operational errors that cost money no matter which way the market goes. They come from the calendar, the order ticket, and the tax code. They are boring, and they are almost entirely avoidable.
Each one is set up below as a belief traders tend to hold, followed by what actually happens and the fix.
Six Swing Trading Mistakes at a Glance
| Mistake | Where it bites | Fix |
|---|---|---|
| Reading the ex-dividend drop as a signal | Charts, gap scans | Check the dividend calendar first |
| Market orders outside regular hours | Pre-market and after-hours fills | Limit orders only |
| Treating the after-hours print as “the price” | Overnight P&L, stop decisions | Wait for the opening auction |
| Forgetting holidays and half-days | Thanksgiving week, Christmas Eve | Map closures before you enter |
| Rebuying within 30 days of a losing exit | Taxable accounts | Track wash-sale windows |
| Mixing up SPY, SPX and ES numbers | Pre-open “futures are up” reads | Convert before you compare |
Mistake 1: Treating the Ex-Dividend Drop as Information
The belief: every SPY gap tells you something about sentiment.
The reality: four times a year SPY opens lower for a purely mechanical reason. On the ex-dividend date the share price is adjusted down by roughly the dividend amount, because new buyers no longer get the payout. SPY’s most recent ex-date was September 18, 2026, at $1.88883 per share, and with a trailing yield near 1% each quarterly payment is worth roughly a quarter of a percent of the price. A quarter-percent “gap down” on a quiet morning is not the market voting on anything.
Two more wrinkles. If you are short SPY over the ex-date, you owe the dividend to the lender. And unadjusted charts show a fake gap that adjusted charts erase, so two traders can look at the “same” chart and see different setups. The next SPY ex-date should fall in mid-December, in line with the quarterly pattern. Confirm it against the fund’s own calendar rather than guessing.
Mistake 2: Sending Market Orders Outside Regular Hours
The belief: SPY is the most liquid ETF in the world, so a market order fills fine at any hour.
The reality: SPY is deep from 9:30 to 4:00. Outside that window there is far less of it. The SEC’s investor bulletin on extended-hours trading lists the risks plainly: less liquidity, wider bid-ask spreads, more volatility, and order handling that can differ from the regular session. It also notes that many brokers accept only limit orders in extended hours, which tells you something about the risk.
The fix: use limit orders for every pre-market and after-hours trade, and decide your limit price before you open the ticket. If you can’t name a price you’d be happy to get, you aren’t ready to trade in that session.
Mistake 3: Treating the After-Hours Print as the Real Price
The belief: if SPY printed 0.6% lower at 7:15 p.m., you’re down 0.6%.
The reality: the same SEC bulletin warns that extended-hours prices may not reflect where the stock trades in the regular session. It also notes that trading venues are often unlinked, so different systems can show different prices for the same security at the same moment. A thin evening print is one data point, not a verdict. Traders who panic-sell on it often lock in the worst price of the night.
The fix: decide before the close what overnight move would actually change your plan. Write it down, then judge the morning against that note instead of against whatever is on the screen at 2 a.m. This kind of pre-commitment is where a structured input, like SpyTomorrow’s nightly pressure reading, earns its place. It doesn’t predict the open. It makes you state your expectations while you’re still calm.
Mistake 4: Holding Into a Holiday Without Looking at the Calendar
The belief: an overnight hold is an overnight hold.
The reality: some “overnights” are much longer than others. The rest of 2026 has four schedule changes worth marking now, according to Fidelity’s 2026 market holiday calendar:
- Thursday, November 26 — closed for Thanksgiving
- Friday, November 27 — early close at 1:00 p.m. ET
- Thursday, December 24 — early close at 1:00 p.m. ET
- Friday, December 25 — closed for Christmas
A position held from Wednesday’s close before Thanksgiving carries about 41 hours of news risk into a thin half-day session. A Christmas Eve hold runs into a three-day weekend. None of this is hidden, but it is easy to forget when you plan one night at a time. If gap risk is already your main exposure, the gap math on why stops don’t protect you overnight gets worse when the gap covers two sessions of news instead of one.
Mistake 5: Re-Entering Too Soon After a Losing Exit
The belief: you closed at a loss, so the loss counts for taxes.
The reality: in a taxable U.S. account, if you buy the same or a “substantially identical” security within 30 days before or after a sale at a loss, it’s a wash sale. The loss is disallowed for now and added to the cost basis of the new position. Overnight traders who exit SPY on Tuesday and re-enter on Thursday can stack up a year of wash sales without noticing.
The loss isn’t gone forever; it’s deferred. But traders who expect to offset gains in December are often surprised by what their broker’s 1099 actually shows. Keep a column for wash-sale windows in your journal, and have a tax professional confirm how the rule applies to your situation.
Mistake 6: Comparing SPY, SPX and ES Numbers Without Converting
The belief: “ES is up 20” means SPY will open up about 20 cents, or about 20 points, depending on who you ask.
The reality: these are three different instruments with different scales. SPX is the index. SPY trades at roughly one-tenth of it and pays dividends. ES futures trade above the index by a fair-value spread that shrinks toward expiration. A pre-market read that skips the conversion can turn an “up” morning into a red SPY open. We walked through the arithmetic in how ES fair value changes what “futures up” means for SPY’s open, and the overnight trading terms glossary covers the vocabulary if any of these labels are new to you.
A 60-Second Pre-Trade Operations Check
None of these mistakes need a better market view. They need a short checklist you run before the order goes in:
- Is an ex-dividend date, holiday, or early close inside my holding window?
- Is my order a limit order if it fills outside 9:30–4:00?
- Have I written down the overnight move that would change my plan?
- Did I sell this, or something substantially identical, at a loss in the last 30 days?
- Am I comparing SPY to SPY, not to SPX or ES?
Direction is the part of trading you can’t fully control. Operations is the part you can, and it compounds just the same.
This article is for educational and informational purposes only. It does not constitute financial advice, investment recommendations, or a solicitation to buy or sell any securities. Past performance and historical data do not guarantee future results. All trading involves risk, including the potential loss of principal. Always do your own research and consult with a qualified financial advisor before making trading decisions.
