These occurrences can happen at any time, but are most common during periods where markets are volatile. Since cryptocurrency markets are inherently more volatile, we can expect more fluctuations in price and therefore potential for slippage. slippage, however, does not denote a negative or positive movement. There are two types:
Positive slippage means that your order was filled at a favourable price. For example, submitting an order to buy at $100, but it gets filled at $98. Many traders consider these "virtual profits".
Negative slippage means that you bought or sold at a worse price than displayed on the quote, Or worse, you missed the opportunity because price moved against you. It’d be like trying to sell at $110, only for the price to move lower before your order goes through. A market order would at least get you out of the position, but at the risk of selling for say $107.