Quick version:
Girlfriend's car ('96 Civic EX) was heating up when not moving. Slow, steady coolant loss. No visible leaks.
Rad. fan would only come on with AC. Easy fix, ECT switch.
The civic was operating normally after the fix, but still lost coolant. I added leak detection dye, and was planning to test her car soon. She was driving, and began to heat up even while driving.
Fan was on. She left it running, popped the hood, and the overflow was overflowing and bubbling. After sitting to cool, radiator was near empty.
She happened to be near her family's mechanic, and so took it to him. He diagnosed a head gasket failure, and wanted about a grand for the fix (including valve job). Not a bad price, but still a grand.
I offered to do the fix. Mechanic said he'd checked compression, and found cylinder 3 low. This, combined with an over-pressurized cooling system, and steady coolant loss with no visible leaks sounds like a HG failure to me too.
So, I take the thing apart and the head gasket appears intact. There is no "steam cleaning" effect on the cylinder in question.
I made a huge mistake by not performing my own compression check before tearing it apart.
Here's my question for you guys. Do you regularly see HG failures where there is no obvious sign of the failure itself? The two I've seen (fixed) were of the "spectacular" and obvious type.