Quite a few spring to mind:
SE satio - why, why why why did SE release this phone in the state it was in? It was slow, buggy, the UI was ridiculously backward (they somehow managed to ruin symbian OS even more) and it was way too expensive. Sure it had a nice camera, but other than that it was just horrid to use.
SE C905 - much the same with the satio except this seems to have developed more problems later in life as I keep having people come back into the store with them to send them off for repair with the same issues over and over.
Any samsung touch screen with a resistive screen - they're horrible to use. Often ending up with you selecting the wrong thing and the tocco and tocco lite have that stupid side bar thing which when you try and select something, just ends up with you dragging loads of crap onto your home screen. Thankfully with the wave/monte/galaxy they use capacitive screens now which are much easier to use.
LG shine - stupid scroll bar thing that liked to stop working randomly and ended up with you selecting the option above or below what you wanted when you click down as it would move when you did.
T-mobile pulse mini - it was good on paper with android 2.1, a 3.2mp camera and being at £99 on prepay; in reality the screen is hideously pixelated, it's unbelievably slow and the camera is just awful. The screen is also too small for anyone with normal sized fingers to use the keyboard on. Bring back the normal pulse plz
It's quite handy working in a mobile phone shop
Btw, skyfall, yes we hate the loan phones too, they're changing them for nokia 6303's in the future thank god!