Sometimes when I finish some tough cardio, my legs are all wobbly and it feels like I might fall over if I keep on standing. Sometimes I'm climbing stairs (not a stair climber) for exercise and near the end my legs feel a little loose, I wouldn't want to fall down a flight of stairs from over-exercise. I was wondering if anyone passed that threshold and if any distinct warning signs come before the actual bonk.

You sound like you are just pushing your body to the limits - which (with a couple qualifiers) is perfecly fine. I can finish a tough interval or training session and can hardly stand. the important thing is to let your body recover after the session before you attempt this type of overload again.
"The Bonk" is so named because you really can't feel it coming. Your body has simply run out of energy, pure and simple. The only way to prevent this is to adequately fuel before during and after your training.

Never actualy happened to me but I got close a few times, starting to feel faint. On of the trainers at my gym has told me that she has several times. she says its due to working too close to your maximum heart rate for too long.

Sometimes when I finish some tough cardio, my legs are all wobbly and it feels like I might fall over if I keep on standing. Sometimes I'm climbing stairs (not a stair climber) for exercise and near the end my legs feel a little loose, I wouldn't want to fall down a flight of stairs from over-exercise. I was wondering if anyone passed that threshold and if any distinct warning signs come before the actual bonk.

Sadly, I have :o

I was doing deads one day and I just passed out right there on my last set, last thing I remember was dropping the bar, a loud thud and then about 5 people around me trying to help.

I did feel a rush of blood to my head just prior to starting that last set, and I did feel just a little dizzy, but stupidly I thought I could just finish real quick and take a min to relax.

never had that to happen, but i've felt like it could happen

Yes, I bonked once on a really long, tough hike. I only drank water and did not eat for like 5 hours and then right on top of a mountain I just ran out of fuel.

It was really strage, I just felt no energy, was very lethargic, did not even want to move. No disiness or physical pain. Then I ate a lot and drank a lot and felt much better in half an hour.

Since then I am really careful to always eat/drink enough on long "endurance" events.

I've never completely passed out, but I've definititely been on the verge. I start to feel weak, and I kind of see "white". I always stop immediately and walk around to let my heart rate come down. Then I eat something.

I asked a trainer about this. It doesn't happen too often, but often enough for me to be concerned. She told me that I am either dehydrated or I don't have enough carbs in my system. The carbs provide the energy for the workout. She recommended that I drink a glass of orange juice or something before I exercise.

Debu, do you eat before your cardio sessions?

There are two distinct issues here.

The first being that wobbly feeling. This happens to most of us who push ourselves in the gym. You get that feeling mainly because you've depelted the muscles store of energy and haven't given it an ample time to rest and recuperate (Mainly because you're still standing and or walking around).

Depending on the intensity of the work out and how much adrenaline your body produced, your body could also be recovering from a post fight or flight response.

Secondly about feeling light headed issue. This can be a combination of both improper breathing, and diet. Most commonly this will happen when lifting heavy weights and not exhaling on the press.

This happens because :
-Blood is being rushed to the muscles instead of the brain ( read oxygen)
-Pressure from pressing such a heavy weight being compressed in your lungs which leads to :
-excess carbon dioxide in your body

Your body knows how to keep it self alive, if your doing something that it thinks is really really bad, it'll just shut it self down for a bit until everything is normal.