What happens when a creature can neither maintain nor cease concentration?

Question

A number of effects in 3.5 render a creature unable to maintain concentration on a spell. A number of other effects render a creature unable to take free actions, thus preventing a creature from ceasing concentration on a spell. The nauseated condition, in particular, does both, specifically stating both that a creature subject to it is unable to maintain concentration on spells, and that such a creature is unable to take any actions except a single move action once each turn.

If a creature is unable to cease concentration, what happens when they are unable to maintain concentration?

Answer 1

The fundamental misunderstanding in the question is the assumption that ceasing concentration requires a free action. As per the SRD, emphasis mine:

Cease Concentration on Spell

You can stop concentrating on an active spell as a free action.

This states that you may use a free action to cease concentration on a spell. It does not state that a free action is required to cease concentration. The use of a free action is just one of the ways that you can cease concentrating on a spell, in this case deliberately.

Concentration also ends if you fail to use a standard action to maintain it on your turn (no action required on your part, only the lack of one). You'd also lose concentration by failing a reactive concentration check as per the Concentration skill, and by RAW that use is technically a free action, so you'd auto-fail if you have a condition that precludes you from taking free actions.

In the event that you are subject to a condition which explicitly precludes you from concentrating on spells, concentration ends the moment the condition applies to you. If you had a condition which prevented you taking standard actions but did not explicitly preclude concentration, concentration would end on your turn when you fail to take a standard action to maintain it.

Answer 2

Ceasing to concentrate on a spell is an action. Failing to continue concentrating is not an action, but rather an effect of something else. If something (like being afflicted with the nauseated condition) makes you unable to continue concentrating or forces you to lose your concentration, then you stop concentrating and the spell ends according to its text.