I'm always doing this! I now set a timer on my phone to remind me when something's done! I did what you did with pear, & ended up binning it, as the pear smelt all smoky & yuck. I don't think it would be harmful to eat though, just maybe not very tasty.
I have just had sweet potatoes on in the steamer for dc2 who is now 8 months old, quite abit to mash and freeze. However silly me forgot all about it, and whilst upstairs ironing I smelt something burning and rushed downstairs. The steamer pan which contains the water has gone all black after the water evaporated, however the potatoes look fine and taste ok (I think, weird tastebuds can never tell).