• Different implementations of TeX and its kin, by design or oversight, force different choices with respect to: $\operatorname{Blah}, \operatorname{Blah},$ on the one hand, and $\operatorname{Blah}, \operatorname{Blah}$, on the other hand, at least, if you want to avoid the risk of having a punctuation badly split to the next line every now and then.