The other day I saw a Budweiser beer sign whose red bowtie neon tube appeared to be rectifying. It seems to shimmer at 60hz just like I remember failing slimline lamps doing. I've never seen neon/argon tubes do this before.

Can anybody explain?

I believe I saw rectifying neon tubes somewhere, too although rarely. I cannot explain the phenomenon, though. Like fluorescents, it might have to do with depleted emitter from one electrode, if these tubes use such emitter material. Just a thought...

I would ask in another way: Why they should not? Andy difference between electrodes causing different cathode fall would lead to rectification:
When they are using hot cathodes, it may be, indeed, depleted emission material from one side.
In case of the hollow cathode concept it may be damaged (bended,...) shape or incorrect position of one electrode
It may be contamination of one electrode.
Or the flicker may be caused by arc instability (so no real rectification), some ballasts are more sensitive to this (C-D multipliers were used here as "energy saving" NS ballast, they had way lower losses then regular NST; these supplied the tube string by DC)...

@Medved: I'm not sure why they would not, I only know I've never seen it before. Years ago I used to work for a sign company, and we made and serviced numerous neon signs. Many of those we took care of were decades old, and many of the original tubes still worked well, so I assumed that the electrodes didn't use emissive material like fluorescents. At least in my experience, they electrodes themselves didn't wear out. When an individual tube was out, the problem was usually caused by a crack in the glass, due to thermal shock from water, etc. (Or from a highly inexperienced tech's (Me ) poor-quality tube rebuild.

I'm not sure if the Budweiser sign used the typical magnetic transformer or one of the modern electronic types. If I get a chance I'll try to get a better look at the sign.

It sure looked like typical fluorescent tube rectification to me...

Hollow cathodes (I guess the mostly used type for long tubing) last indeed forever, but if they are deformed or badly placed during tube assembly, such tube is more likely to rectify.
Hot electrodes are then sensitive to any contamination and the final activation step (if not done properly, the emission layer does not work)

final activation step ?

It seems that typical "EOL" of neon signs in Israel, is driver malfunction (as wild flashing, which are most common in neon signs in Israel, turned out to be a driver malfunction and neon signs in Israel opreated at HF, as they don't flickers, not even through my camera) and breakage because of a wild weather and vandalism.

I think this "wild flicker" is when a wire gets loose and is arcing (in series with the tubes), or maybe when isolation breaks and it is arcing to the metal parts of the sign or to the building wall (in parallel with the tubes)

If so, the arcs must be pretty small since it is rare to see them (the few i seen were series)

Here is my video of a neon sign of the optic shop of "Optica Halferin" (אופטיקה הלפרין) chain store in the center of Kiryat Ata, flickering.
Btw, i also saw such flickering, with F96T12 8ft Philips ALTO HO T12s 110W fluorescents, operated with Eltam rapidstarts.

@dor123, usually that type of flickering happens to 120v neon signs on magnetic transformers when the transformer develops an internal short. I've never seen that type of flicker caused by tube failure. In the case of the sign, the flickering was much more subtle.