Sandwich attack

A sandwich attack is the most common harmful form of MEV. A bot sees your buy before it lands in a block and places its own buy right in front of it. The price rises, so you buy at a worse price, within your slippage setting. Right after your transaction the bot sells at the higher price. Your swap sits in the middle like the filling between two slices of bread.

The bot's profit is your loss: you receive fewer tokens than the quote promised. The attack pays best when the slippage setting is high and the pool is shallow. That is why setting slippage to tens of percent so a trade “definitely goes through” is, in practice, an invitation to be sandwiched.