How you instruct a broker determines what happens when conditions are not calm.
Market orders
Executed at the best available price immediately.
Which guarantees execution and not price.
Limit orders
Executed only at a specified price or better.
Which guarantees price and not execution.
Stop orders
Triggered when a price is reached, then becoming market orders.
Which can execute far from the trigger in a fast market.
Stop limit orders
Combining both, with the risk of no execution at all.
Where the choice actually bites
In a fast-moving market, a market order can execute far from the price you saw, and a stop order can trigger and then fill much lower.
Which is exactly the situation people use them to protect against.
Limit orders remove that risk and introduce a different one: the order may simply not execute while the price moves away.
Gapping
Prices jumping between levels with no trading in between.
Which defeats stop orders entirely.
Trading outside main hours
Thinner liquidity and wider spreads.
Which makes market orders more dangerous.
Broker execution policies
Published best execution arrangements.
Which are worth reading once.
Practical guidance
Limit orders for anything illiquid or volatile.
The situations where it matters most
Illiquid shares, market opens, periods immediately after news, and any instrument with a wide spread.
Which are exactly the moments when a market order can execute at a price you would never have chosen.
Using limit orders as a default costs occasional missed trades and prevents the category of outcome people find hardest to accept.
Good till cancelled and day orders
How long an order remains live.
Which brokers implement differently.
Auction participation
Orders included in opening and closing matches.
Which can achieve better prices for larger trades.
Fractional shares
Platform-specific execution arrangements.
Which are not the same as exchange trading.
Reading your broker's terms
Order handling and execution policy.
A default that works
Use limit orders, set them at or near the current quote, and accept that occasionally the trade does not happen.
Which removes the risk of executing far from the expected price.
For anyone investing regularly into liquid funds this rarely matters; for anyone trading individual shares, especially smaller ones, it matters a great deal.
Automatic investing
Regular purchases executed by the platform.
Which typically use scheduled aggregated trades.
Stop losses in practice
Protection that fails in exactly the conditions it was set for.
Which is worth understanding before relying on one.
Overnight news
Prices gapping at the open.
A general note
Order handling differs between brokers and venues; the broker's policy governs.
Why market structure is worth understanding
Most investment writing is about what to buy. Very little of it is about how the machinery underneath actually works: how orders are matched, how funds track what they claim to track, how prices respond to information, and what regulation does and does not cover.
That machinery determines a large part of the outcome. It sets what trading costs, it explains price movements that otherwise look irrational, and it defines the boundary between a risk you accepted and a failure someone else is responsible for.
None of it tells you what to invest in. It tells you what is happening when you do, which is a different and more durable kind of knowledge than any particular recommendation.
Where the reliable information is
Exchanges publish their rulebooks. Index providers publish their methodologies. Fund managers publish factsheets and annual reports. Regulators publish investor education material and firm registers. Companies publish annual reports and prospectuses.
All of it is free, all of it is authoritative, and almost none of it is promoted, because nobody makes money from you reading it. It is consistently more useful than commentary about it.
A general note
This article describes how markets and financial products work and is not investment, tax or financial advice. Rules, protections, tax treatment and market structure differ substantially between countries and change over time. Anything with money attached warrants checking against the rules applying where you are, and where a decision is significant, advice from a qualified professional who knows your circumstances is the appropriate route.
The recurring lesson
Across almost every topic here, the same pattern appears: a mechanism that looks arbitrary from outside turns out to follow published rules, and the rules are available to anyone who wants to read them.
The gap between what is knowable and what most investors know is not caused by secrecy. It is caused by the material being dry, unpromoted and available only to people who go looking for it, while a great deal of louder content competes for the same attention with considerably less substance.
Nobody needs to become an expert in market microstructure. Knowing that the answers exist, and roughly where, is enough to avoid the specific mistakes that come from assuming there is no answer.