[adamwathan]

Borders still affect the positioning of things even with box-sizing set to border-box. For example, here "Block 1" is still two pixels taller than "Block 2":

<div style="padding: 8px; border: 1px solid black"> Block 1 </div>

<div style="padding: 8px;"> Block 2 </div>

https://jsfiddle.net/t4rkvfzx/

[GrantSolar]

Does that really matter though? A difference of 2px for something temporary doesn't seem like much concern. While I understand the implication is that these can accumulate and cause larger layout issues, the purpose of this practice is to highlight those areas to remove that accumulation anyway

[ComputerGuru]

The bigger issue is that you don't know if a UI inconsistency is a bug or a side-effect.