It's handy to be able to diff the output of two commands by using process substitution [1].  For example, I've often used the following command to make sure that the result of my merge conflict resolution looks good with respect to the patch being rebased in git:

    $ meld <(git diff --cached) <(cat $(git rev-parse --show-toplevel)/.git/rebase-apply/patch)

(looking at it now I think I can get rid of the cat altogether, but I digress).

Here's another simple example:

    $ meld <(echo hello world) <(echo hello pizza)

This has worked for years for me using meld, but recently it has started erroring out with a little error pane at the top that says:

    There was a problem opening the file "/proc/self/fd/11".
    Not a regular file.

[1] http://www.tldp.org/LDP/abs/html/process-sub.html