Pure Mortals and Items of power

[Sanctaphrax]

Yeah, I noticed that contradiction, too.  Then after letting it stew for a bit, I think I've come to the conclusion that maybe its more a matter of being worded unfortunately rather than being contradictory.  Basically, I think that the last quote was intended to set forth an exception for potions only to the rule about spending a mental stress to gain an extra use of an enchanted item.  So in other words, yes, a potion can have multiple "servings", but once the potion is used up, it's done, and you can't use it again by spending mental stress.

This is also my interpretation.

[Tedronai]

That does seem more consistent than the interpretation I came up with.

[GryMor]

A frequency specialization allows you one more use
per session. A strength specialization increases
the effect strength of your basic enchanted items
by 1 (note, any increases to the basic strength
are applied before cutting it in half for an alwayson
item). In the case of potions, this can create
stronger potions, or ones that you can get two
uses out of.

That is odd, my copies (pdf and physical) doesn't have all of that text, instead I have:

A frequency specialization allows you one more use
per session. A strength specialization increases
the effect strength of your basic enchanted items
by 1 (this strength specialization bonus can’t
be traded in for an additional per-session use).
In the case of potions, this can create stronger
potions, or ones that you can get two uses out
of.

Where did

(note, any increases to the basic strength
are applied before cutting it in half for an alwayson
item)

come from?

[Tedronai]

Dangit, I quoted from my pre-release again.  That's where it came from.

[Radecliffe]

You should burn that copy or at least put it someplace it can't hurt anyone.