mempool congestion, the transaction you send out is like going to the hospital for registration: queuing first, and there will still be people cutting in line (paying extra). You think clicking confirm is the end, but actually you're waiting for miners/ordering algorithms to pick you, if gas is too low it stays stuck, and after a long time sometimes you’ll be “replaced” by your own “bidding higher,” or simply expire. What's more annoying is that during this period the price changes, when slippage triggers, the DEX directly rolls back, and the transaction fee is still charged… To put it simply: you’re betting on whether the “next block” is willing to accept you. Recently, the L2 hype battle is more about TPS/fees, but I care more about whether the ordering rules during congestion are transparent or not, and how reorganizations/delays are handled, otherwise if the oracle updates are a bit slow, subsequent liquidations/arbitrage will all get messed up. Anyway, during peak times I’d rather do less, and if I really need to act, I keep an eye on pending, don’t pretend to sleep.