I wouldn't really consider this a screwup on your part. Sure, you probably didn't do everything perfectly, and the senior dev probably should have caught the potential problem. What went wrong, though, was a failure of the development process and the accumulation of technical debt. You should never have been able to introduce a problem on that scale to begin with. You might have still managed to introduce a bug, but it should have been minor at worst.
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I wouldn't really consider this a screwup on your part. Sure, you probably didn't do everything perfectly, and the senior dev probably should have caught the potential problem. What went wrong, though, was a failure of the development process and the accumulation of technical debt. You should never have been able to introduce a problem on that scale to begin with. You might have still managed to introduce a bug, but it should have been minor at worst.