Problem with 40D? Light and dark images

I have a new 40D that I put through it's first wedding today and I noticed something very strange and alarming. I would be in manual mode and shoot a series of shots in a span of a few seconds with the same composition. Same settings, same everything, but a couple of the shots would be drastically darker than the others. This was also confirmed by the histogram so it wasn't just the LCD. I'm talking like a stop or more difference here.

The only custom functions I have on are highlight tone priority, ISO expansion and focusing with the AF on button. Everything else is default.

Is this a know issue with this camera? What could be causing this? Could it be I'm doing something I don't realize?

I have had a 30d for a long time (still do, shot it today too) and never seen anything like this. Today was the start of my wedding season and I have several coming up. I'm hoping maybe the experienced people here can offer some suggestions on what I can do that hopefully doesn't involve me being w/o a second camera for long.

Chris

Show us the pictures with EXIF: one with the correct exposure, the other one darker.
Only that way we can hopefully come up with an educated guess what could be the cause for under exposure.

These two were shot a split second apart in a burst.

check if your AEB is on? auto exposure bracketing?

Nope, just checked it again to make sure.

Your metering mode is set to Pattern. Not sure what that is because the 40d guide shows only four: Evaluative, Partial, Spot, and Center-weighted so I'm not sure what mode "pattern" is (sounds like it might be either the Evaluative or possibly the Center weighted).

In any case I usually shoot in either Partial or Spot.

First shot metered on the dress the other on the background?

If the OP is using Manual mode the metering will have no effect on the shots once the settings are 'dialed-in' - Thats the point of using Manual

I assume the light didn't change in those few seconds?

I suspect its "Highlight Tone Priority". Try a similar test without it.

Its shifting the tone curve to keep the white dress within exposure as its supposed to. The detection must be a bit hit and miss.

I'm thinking metering. Are you set to spot metering? I don't know what "pattern" is either.

kenyc wrote in post #5757896
I'm thinking metering. Are you set to spot metering? I don't know what "pattern" is either.

Pattern is Evaluative.

The first shot look exactly like HTP in action but what caused camera to switch from no HTP in the second shot (chronologically first), to HTP in the first shot, in 1 second, is beyond my comprehension.

Does the metering even matter if he is in manual?

No.

arkphotos wrote in post #5757995
Does the metering even matter if he is in manual?

True, it shouldn't.

There is no discrepancy in metering: both shots are f/4, 1/250, ISO 200.
Yet, the difference is almost 1 stop..