What is happening
You clicked at one price but the trade opened or closed at a less favourable one. In most cases this is normal market mechanics — the spread and slippage — rather than a mistake, but it is worth confirming which.
Common causes
The spread: you buy at the ask and sell at the bid, so a chart showing a single line will look 'off' by the spread. Slippage: in a fast market, price moved between click and fill. A market order filled at the next available price rather than the last shown. Or a stop that triggered and filled beyond its level during a sharp move or gap.
How to check
Compare the fill against the correct side of the market (ask for buys, bid for sells) at that instant, not the single chart line. Look at whether the market was moving fast or gapping. For a triggered stop, expect a fill at the next price, which can be worse in volatility.
How to reduce it
Set a maximum deviation on market orders, use limit orders when you need a precise price, and avoid trading in the most violent seconds of news. Trade liquid instruments in busy hours for the tightest pricing.
Broker's fault or yours?
The spread and occasional slippage in fast markets are normal. If your fills are consistently worse than the true market on the correct side, or one-sided against you, that points to an execution problem worth investigating and comparing against another broker.