Canon 420 Speedlite overexposure problem

I am baffled by sometimes getting photos flooded with light, so over-exposed they are not usable at all (other times under-exposed.) Is there some setting I have accidentally pressed that is causing this? It is intermittent--maybe 5-10% of the time only. It occurs in darkened rooms when I am not very close to the subject. I have a Canon Rebel XT with 420EX Speedlite. Thanks in advance for any enligtenment!

One typical time when you see this is at a wedding with a white bridal dress and a black tuxedo. The difference in reflectiveness of the subject can throw off exposures quite a bit. Check to see if your subject had a lot of black or white.

---Bob Gross---

I have since checked my settings and still can't figure out what caused this. Example:

1) Photo#1: 1/15 & 4.0 aperture, ISO 400 - badly over-exposed
2) Photo#2: 1/15 & 4.0 aperture, ISO 400 - slightly under-exposed. Same setting exactly, fairly dark room, both photos about 12 feet away from subject

I am not sure whether they were taken on the "P" or automatic setting. Both appear to have the same settings but the histograms were totally different. Any thoughts?

Carole32 -- your images were not included in your post...

If you look in the EXIF data for each image, you will see whether it was P or any other mode, and exactly what the settings were. All that tells you is how the camera thought it was setting everything. However, if some weird thing happened, e.g. if the lens malfunctioned, then all bets are off. If a lens was supposed to stop down to f/4, but if it were "hanging" and stayed wide open at f/1.8, then that would explain an overexposure. All you know for sure is that the histogram, after the fact, showed the overexposure, but you don't yet know what mechanism led to that.

If a Speedlite is not fully charged up to fire, then sometimes it fires with a low power, but that does not explain your symptom, which is the other way. If you had a loose data connection between the camera and the Speedlite, then that would explain a few things. If you have a Canon friend to help, move the Speedlite to his camera and move his Speedlite to your camera. If the symptom starts moving around, that suggests that it was your Speedlite in trouble. If the symptom does not move, then that suggests that it was something closer to your own camera. Sometimes your local Canon dealer can be a friend for something like this.

---Bob Gross---

Carole,
Does the pop-up flash on the camera give you similiar inconsistant results?
As Bob suggested, check the EXIF data, it seems a bit odd you may be getting 1/15 sec shutter speeds with the flash in P or Auto modes.

I was getting massive overexposures the other day, it turned out that the flash wasn't seated on the camera properly.

Check to make sure your flash is fully charged before you take a picture, that's one obvious culprit.

I'm with tim... fresh batteries and make sure the flash is seated properly in the hot-shoe (lock it down). Also, it can give you different output depending on what type of metering you use and where you point it to meter.

Wow, so many diverse responses. I could be the batteries which should have needed changing by now. I will change them and see if the problem persists. When this happens, the shutter feels like it's open an inordinately long time which I'm sure is what's causing the problem. I took many shots after this happened (it persisted for about 10-20 shots and then went back to normal. I rfeally appreciate everyone's input!