how is early exercise a risk with box spreads?

B

bmoney

consider I buy the following spread:

buy a call and sell a put at strike of 1
sell a call and buy a put at strike of 3
net debit paid is $190, leaving $10 of profit upon expiration

If the stock rallies from $2 to $10 and my $3 strike call is exercised early, couldn't I simply counteract that by early exercising the call I purchased at $1 strike, making the 200 dollars from that for the $10 profit and then sell the remaining puts/let them expire OTM? I'm failing to understand why early exercise is such a risk
 
This is how some people blew up their account, including famous 1R0NYMAN.
https://www.reddit.com/r/path%3D%252Fr%252Fbestof%252Fcomments%252Fah8lnm%252F

The main issue is that profitable box spread is simply not possible because market makers price all options appropriately to prevent arbitrage.
The only time when a box appears to be cheaper than it should be is when a stock has very high %interest rate for shorting it. So you'll get assigned 100 short shares overnight and have to pay huge %interest on it by the morning, or for 3 nights if assigned on Friday. The other party will earn overnight %interest by holding the shares they obtained through option/call exercise.
 
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