This may be an absurdly stupid question, but I gots ta know!

Handlebar 26.0mm, and the stem I want is 26.2mm.

Am I going to have a problem with this?
Is that .2mm going to mean that I'll need to put some sort of a shim in?

Thanks!

You're gonna need to put a sliver of a soda can in to make it work. .2mm shims don't exist.
-Gene-

The proper shim would be 0.1mm (0.004"), half the difference in the diameter.

Aluminum pop cans are cheap, plentiful and the right gauge (at least the local Pepsi cans are), however I'll mentions some other options, for future reference. Shim stock is available that thin, though you may have to go to an industrial supply house and they may not sell you a small piece. Often, machine shops have a good supply of shim stock in various gauges and are willing to sell (or give) you a small piece. Finally, some automotive shops sell individual feeler gauges, which make excellent shims, but may not be large enough for some applications. In this case you could use two, placed side by side. Feeler gauges are excellent for shimming things like loose crown races and headset cups.

Looks like I'm cutting up a pop can.

Thanks guys!

From what I understand, a Yuengling can is the best. I volunteer to provide one.

On a serious note, I am currently using a 26.4 Cinelli stem (I've never heard of a 26.2) with a 25.4 bar.
The shim is cut from an old sleeve and it looks really thin, until I try to put it in, and then, though it made it OK, it looks huge.
The gap of the stem where the triangular gasket fits is way too big, so I've got to go to plan E.