Most post-mortems of a losing overnight trade start in the wrong place. The trader reopens the chart, rereads the thesis, and asks what they misread about the market. Sometimes that is the answer. More often the read was fine and the loss was manufactured somewhere else entirely — in the size, the order type, the calendar, or the way the trade got graded afterward. None of that lives in the analysis. All of it lives in the trade log.
That is what makes these particular swing trading mistakes so durable. A bad read announces itself: you were wrong, the market said so, you move on. An execution error looks like ordinary bad luck at the time and leaves no obvious scar, so it survives to run again next week. Below are six that show up in your records rather than your reasoning — each paired with the specific field in the log where you would actually find it.
The log knows things the thesis does not
A thesis is a story you tell once. A log is a record you cannot argue with. If you keep even a minimal one — date, direction, share count, entry, exit, and a sentence of reasoning — patterns emerge within a few dozen trades that no amount of chart review will surface. The most common one is not a market misjudgment but an inconsistency — the same trader, running the same setup, behaving differently depending on mood or how the last trade went.
The six swing trading mistakes at a glance
| The mistake | Where it shows up in the log | The correction |
|---|---|---|
| Sizing by conviction | Share counts swing wildly with no relation to stop distance | A fixed dollar risk per trade |
| One size across every regime | Constant share count while daily range doubles | Size in units of risk, not units of shares |
| Market orders into the open | Fills noticeably away from the printed opening price | Limit or market-on-open orders |
| Adding to an overnight loser | A second entry below the first, absent from the original plan | Pre-commit to the add, or forbid it |
| No calendar check | Losses clustering on release days and earnings dates | A 60-second calendar review before every hold |
| Grading by outcome | Winners are never reviewed at all | Score plan adherence separately from profit and loss |
Two mistakes in how you size
1. You size by how good the idea feels
Open the log and sort by share count. If the largest positions are the ones you felt best about rather than the ones with the tightest invalidation, you are sizing on confidence. Barber and Odean’s study of 66,465 discount-brokerage households from 1991 to 1996 found that the most active traders earned 11.4% annually against a 17.9% market return, with overconfidence the mechanism they identified. Confidence is not a risk input — it arrives before the outcome and correlates poorly with it. The correction is unglamorous: decide what a single overnight trade is allowed to cost you in dollars, then let that number and your invalidation level set the share count — which is the same reason your stop loss is not your real risk number.
2. You carry the same share count into a different market
Two hundred shares is not a fixed amount of risk. It is a fixed amount of exposure, and the risk attached to it changes every week as the market’s range expands and contracts. A trader holding the same position through a calm July and a violent October has quietly doubled their overnight risk without ever deciding to. The log makes this visible immediately: plot share count against the average daily range at the time of entry. If the line is flat while the range is not, you have found it. Sizing in units of risk means the share count falls automatically when the overnight volatility regime widens.
Three mistakes in how you execute
3. You send a market order into the opening auction
The open is not a normal moment in the trading day, and treating it like one is expensive. NYSE begins disseminating opening imbalance information roughly half an hour before 9:30 a.m. Eastern, and the designated market maker then opens each security into whatever supply and demand has accumulated. A plain market order sent into that process takes the price the auction produces, which on a gap morning can sit well away from the number you saw quoted seconds earlier. If your log shows repeated fills that surprised you, the order type is the culprit, not the market. This is also part of what actually sets SPY’s opening price while you are asleep.
4. You add to a position that gapped against you
This one is easy to spot because it leaves two entries where the plan called for one. Odean’s work on the disposition effect documented that individual investors realize winning positions at a meaningfully higher rate than losing ones, and adding to a loser is that same reluctance with leverage attached. It gets rationalized as a better average price. What it actually does is increase exposure at the exact moment your thesis has been contradicted. Averaging in is defensible only if it was written into the plan before the position existed, with the size reserved in advance. Improvised at 9:31, it is not a strategy.
5. You did not check the calendar
This one is purely clerical, which makes it the most annoying to keep making. Sort your losing overnight holds by date and check them against economic releases and earnings reports. If a disproportionate share landed before an 8:30 a.m. print or a major after-hours report, you are not unlucky — you are skipping a step that takes under a minute. Holding through a scheduled catalyst can be a perfectly good decision. Holding through one you did not know about is not a decision at all.
One mistake in how you grade yourself
6. You score the trade by the outcome
A single overnight session is a very small sample. The trade that worked may have worked despite an oversized position and an improvised exit; the trade that lost may have been executed exactly as planned. Grading only on profit and loss teaches the wrong lesson in both cases, and it explains why almost nobody reviews their winners. Keep two columns: what the trade returned, and whether you did what you said you would. When we ran an audit of 24 of our own pressure readings, even the directionally correct ones would have produced very different results depending on how the trader sized them.
A ten-minute audit of your own log
You do not need thirty trades to learn something. Pull your last fifteen overnight holds and answer five questions honestly:
- Does share count track the distance to invalidation, or does it track enthusiasm?
- Did size fall when the market’s daily range widened, or stay flat?
- How far were your opening fills from the printed open, on average?
- How many positions have a second entry that was not in the original plan?
- How many of your winners have you actually reviewed?
A forecast is a hypothesis about the market. The log is a record of your behavior. Only one of those is fully under your control.
SpyTomorrow’s nightly pressure readings speak to the first of those — how much tension the tape is carrying into the close, so you can size and pre-commit with better information. They are decision support, not a signal to follow blindly, and they cannot fix any of the six mistakes above. Those are yours to find, and the log is where they are hiding.
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.
