Out of my G1 stuff:
The Green plastic seems to go brittle easy for a couple of my gestalt combiners

Sinnertwin: decided I wanted to change his position on Abominus and when moving the small robot head (even holding it as close to the body as I could) snapped in my hand
Vortex: folded out his tail section and the green inner plastic disintegrated in my hand on both sides.
Have since fixed them both with junkers
More recent TF's
Gave my son my Mint in BOX Transmetal Megatron to play with (not knowing he is known for breaking easily). I feel so sorry for my son as he came running to me within minutes sobbing with both Megs arms in his hand. The brown plastic on his chest had disintegrated
Decided to open my BW mutants and put them on display - Soudwave broke in three places just by looking at him

His jaw hinge broke, and where his body comes together chipped in a couple of places just by clicking the two haves together
Finally (sorry to bore everyone) - Decided to open some of my Beast Machines to display better. Started to unwire Nightscream and discovered the clear plastic holding his waist together had broken

so he came out of the box in two pieces

(stupid beast machines

)
surprisingly as 1AZRAEL1 mentioned some of my toys have fallen from significant heights and not a mark on them. The most recent being that I was rearranging my alternators/binaltechs (which are on the top shelf of one of my display cases) Bumped binaltech Arcee an she did a back flip and fell about 8 feet to hard concrete floor

with just that tile stlye carpet (approx .5 cm thick) to break her fall. Not a mark