Practically I think the answer is more or less as Brian says. Mistakes happen and I think it's generally reasonable to simply move on:
[…] if the unintended transmission was brief, within the ham bands and did not interfere with urgent operations.
In fact, I imagine an argument could be made that this is the most reasonable decision, in accordance with §97.101:
(d) No amateur operator shall willfully or maliciously interfere with or cause interference to any radio communication or signal.
As soon as you realize that you were unintentionally transmitting, the first and best thing to do would be to cease transmitting.
The identification requirement is a requirement. I'm not trying to downplay that. But let's say you're in a situation where you've just unintentionally transmitted and perhaps in an interfering matter. Now you have a choice. Which rule would you rather break next — flout the identification rule this time, or perpetuate against the willful interference rule?
Personally, I would rather go with "least said soonest mended". Shut down and be prepared to apologize profusely for the initial unintended (and unidentified) transmission should the occasion arise. If I would willfully keep interfering, solely in an attempt to identify myself, that doesn't seem to help either party.
However, to answer the original question more directly, I think there is nearly always a way to identify — if, only if, and/or when I am reasonably certain that I am able to transmit a clean and non-interfering signal. The "key" (pun intended) is that there is no lower limit to the speed of a CW emission. Quoting §97.119:
(b) The call sign must be transmitted with an emission authorized for the transmitting channel in one of the following ways:
(1) By a CW emission. When keyed by an automatic device used only for identification, the speed must not exceed 20 words per minute;
(2) […other potentially authorized options follow…]
I understand this to mean:
- it is always acceptable to identify by a CW emission, since CW is an "emission authorized" everywhere within the ham bands.
- there's a maximum speed limit when automatic keying, but…
- there's no regulation on speed for manual keying. Neither maximum, or… minimum!
To get to the point, if you have enough control to both start and stop your transmissions at any given time, you are able to identify in Morse Code! Maybe back in ham's heydey they called this "Westinghouse keying" or something :-P