Yesterday I was filming in 25P (Pal) mode when I looked at the screen and saw it had switched to 50i.

This is the second time it's happened to me on this camera.

Has this happened to anyone else?

Yes, this did happen to me a couple of weeks ago, but I didn't notice anything awry until I got back to the studio and saw the clip was interlaced. I NEVER shoot interlaced and couldn't figure out how it happened, so assumed I'd (in a moment of psychosis) inadvertently changed it... but now that you've had this experience too it seems there may be a bug. And I may not be losing the plot after all.

I had something similar happen with my XF305 & I can't for the life of me remember what caused it. I did think that it was changing the bit rate from 50Mbps to 35Mbps made it default to 50i but now I just tried that it didn't change to 50i but I am sure that it was something similar. As the C300 back end is modelled on the XF-series I would assume that it's the same.

Any solutions to this problem? What are Canon saying??

I've only spent a day with the Canon C100, and yes - even that managed to switch from 25p to 50i. I had been diligently powering down for lens changes (of which there were many), swapping modes and PPs, with no ill effect, yet the switch to 50i happened only once. Hmmmm.