emacs 25 choses fonts with different average width for 7x13 font, making bold text take more horizontal space. This is particularly a problem when using themes like
https://github.com/bbatsov/zenburn-emacs which use bold to hihglight matching parenthesis in show-paren-mode, causing characters to shift as the cursor is moved from a parenthesis to a non-parenthesis character.
Repro that demonstrates the problem in both 25 pretest 1 and current head (but is perfectly ok in emacs 24):
$ emacs -Q --eval '(progn (set-face-font '\''default "7x13") (print (face-font "default")) (print (face-font "bold")))'
shows
-misc-fixed-medium-r-normal--13-120-75-75-c-70-iso8859-1
-misc-fixed-bold-r-normal--13-120-75-75-c-80-iso8859-1
Notice the jump from 70 to 80 before the iso8859 component - that's the average width, and that's what I believe is causing bold characters to render wider than non-bold ones. Both are 70 in emacs 24.