physics lab help!?

[star3]

I had to do a lab where we shined a laser on to a wall with different diffraction grattings. since we used different diffraction gratings there were a line of dots a certain distance apart. we had to use the equation
(x/L)=(wavelength/d) to find the wavelength of the laser. So, in my lab writeup, i have to say what the biggest source of error was. but the error cant be like i might have measured wrong, it has to be a error that i cant do anything about. Sorry that its really confusing its hard to explain, but i need some possible sources of error!!!Thanks!

[Sw33tstuff]

The error could be that the laser wasn't properly parallel to the ground, the wall itself, (the wall could have been not such a great vertical area to beam a laser onto) or the material may have affected the outcome by a fractionnnnn..?

haha, and my physics teacher showed this to us, its pretty amazing.
well something like it, i forget but, he showed us this instrument that has two lead balls that rotate and move the two spheres inside the instrument in like a glass case...?

i forget... :/