If it was the nose gear, then somebody forgot the locking pin, usually at least. Northwest went through a spate of that at LGW one time with the centre gear on the DC-10 mainbody.

If it is all the gear down, then it is either the pilots forgetting to pull them all up, or an attempt to lower the brake temperatures and the tyre temperatures. If they suspect that they have shed a tyre, then they might leave the gear down as well, to avoid it getting "stuck up" when shreds of the tyre retract and catch in the wheel bay.

The fuel burn goes up quite a lot if you leave the gear down. If you do not do the calculations properly then :

http://aviation-safety.net/photos/di...0&vnr=2&kind=C

not all types have a warning to let you know that you have not retracted the gear, but you should see the different coloured lights if you have got one dangling.

The greatest cause of gear up landings is......
Amphibians on water.

Happy landings

Spottie