It's a very subjective thing.
Is it ok to take advantage of this software issue in general ? For $20 ? For $500k ? (Leave aside the fact, that LAngel played with "cheated" money).
No. Consciously taking advantage of a software issue is exploiting and scamming at the same time.
Indeed it is a very subjective thing since this scenario cannot be called a scam. It is under the boundary of exploitation. Since one of the definitions of scam is taking advantage of trust or ignorance of a person and steal from them for personal gain