One of my Phals just opened the first flower on its new inflorescence, and
it's missing one of the petals. I am wondering if this is necessarily a sign
of stress, or if this can just happen occasionally?

The Phal seems healthy otherwise. This is a multifloral Phal that still has
many buds on several branches of two spikes, one old spike with new branches
and one new spike, and this anomaly is on the new spike. Since this is the
first flower to open, I do not know whether the other flowers will be normal
this flowering season.

This Phal has flowered twice before in my home. The first time (in April
2004) it flowered it had some anomalies in its flowers (I don't remember
what kind of anomalies exactly), but since this was soon after I got it and
since I found a mealybug on it, I assumed that the anomalies were caused by
mealy related stress. The second time (in March 2005) it flowered normally.

Right now, this Phal has 5 healthy leaves and one lower leaf that is going
yellow (but since it is a lower leaf I assume that it's just aging). The
root system looks ok close to the surface and aerial roots are healthy and
numerous. It was last repotted about 2 years ago (as part of mealy
treatment), but the sphagnum moss still looks fresh enough to me.

What should I do (other than observe it more carefully from now on)? Any
ideas of what can cause such an anomaly?

Thanks,
Joanna