Here are some interesting things I found while making JSW.

Some rooms have objects and bad-objects that are never used, the
drive has a rose growing out the ground but it isn't used, the
attic has 2 leafs as a object to collect but again it isn't used,
The kitchen has a object like the swimmingpools invisable one, which
is similar but rotated 45 degrees.

The elephant's trunk was origanaly made from blocks.

2 monsters are not used, the periscope (used in jsw 2 as marconie 
ted) and what could be either the periscope's wheels or a little 
man with a hat.

One extra room to the right of 'tree root'. (the name is just '[')
Also what appears to be space for 3 more rooms, but is filled with
objects and bad blocks, the names are just nosence ':@{#]'$^$$6' etc.

2 official versions of jsw exist, the original and the bug fix, one has
an extra block in the banyan tree which allows you to climb it. 
Another 'Pre-Release' version is also out there, which has the manic miner
music rather than 'If I was a rich man'.

The swimming pools 'invisable' object is a switch like thing. (it 
should have been a tap as is on the edge of the pool and i think it 
would have been funny) There is also an invisable object in the 
'First Landing' (as well as others) but it has no appearence.

If you do rescue esmarelda you die, apparently she was supposed to
take you to bed and kill you but matt could not be bothered to
code it.

Part of the megatree and the esmarelda room have stairs insted 
of floor. (ie it all acts as stairs see the leafs in 'out on a limb')

If a error ever occured and you went off a screen to somewhere that
didn't exist you would end up at the off licence which is the first
room in the ROM. (this can happen in the 'watch tower' if you jump up 
off the conveyor)

You can't collect all the objects (at least I can't)

The font on the title screen when the ROM first boots is diffrent
from the font used when it scrolls, so when it returns to 
'Press Enter' it looks slightly diffrent.

If you did lay out the map on paper there would be gaps in it, rooms
would not match up with what should be above/below them etc.

If you could get to willys bed you would die, as the pillow is a bad
block, while the bed itself is a right conveyor.

The Room 'Nomen Lundi' and the room under it are actually a plane 
crashed into willys house, thats why its flashing.

'Nomen Lundi' is Latin for 'The Name of The Game'.

It is possable to make willy stick to the ceiling, in the Orangery get
him on the stairs just under the ceiling, then press and hold jump
he should stick to the ceiling.

Interesting bugs involve the frames not being updated correctly when
you fall onto a bad block, willy will die one frame before hitting it.
Try jumping onto the crabs from the ledge on 'The Beach'

For some reason the word 'AIR' is located just before the scroller in
the ROM.

A 'Quirkafleeg' is from a cartoon matt used to read, it would appear to
make lots of people fall down when performed. Or something...

In some of the new JSW ROM's that the game works with it, extra objects
that for some reason the original game would not show, become visable
in my version.

Matt is now 33 and lives in the Netherlands, he never finnished another 
game after Jet Set Willy and left Software projects in 1985.