But are you saying if you put the amount to sell as 50k... when the current price to sell is around 55k... it would be bought off immediately?
If there are enough buy orders at or above the price of $50k, then yes, your sell order at $50k would be filled immediately. But it would be matched against the highest buy orders first and then work down until your entire order had been filled.
I mean... so if someone accidentally puts down they want to sell 5 btc right now on coinbase pro and accidentally put the sell price at 500 dollars... what would happen?
Right now on Coinbase Pro, the price is at $60,950, and there are approximately 5 BTC of buy orders between the prices of $60,900 and $60,950. If you placed a sell order for 5 BTC and $500, you are saying "I will sell at any price above $500." Coinbase Pro would match you against the highest buy orders available, so you would sell your 5 BTC at prices between $60,900 and $60,950.
Lets consider the following example for simplicity. The current bitcoin price is $60,000. There are exactly 1 BTC of buy orders at every $100 increment. So at $59,900 there is someone waiting to buy 1 BTC. At $59,800 there is someone waiting to buy 1 BTC. At $59,700 there is someone waiting to buy 1 BTC. And so on.
You come along and place a limit sell order for 10 BTC at $6000 instead of the $60,000 you meant to enter. You will match against the highest buy order, which is for 1 BTC at $59,900, and so you will sell 1 BTC to this person. You will then match against the next highest buy order, which is for 1 BTC at $59,800, and will sell 1 BTC to this person. You will then match against the next highest buy order, which is for 1 BTC at $59,700. And so on. Your entire 10 BTC will be sold for prices between $59,000 and $59,900. The current price on Coinbase Pro will drop from $60,000 to $59,000. You will have made a loss compared to selling 10 BTC at $60,000 as you intended, but it won't be the catastrophic loss of selling 10 BTC at $6000 because there were plenty of buy orders far above this price which you matched against.
If you did this on a much smaller exchange with poor liquidity, then your 10 BTC may indeed be enough to match against all the buy orders down to very low levels and you would end up with significant losses.