Whenever I press CTRL+S in putty the terminal seems to freeze. It's actually not, but it doesn't show anything. That is because CTRL+S initiates the XOFF command, which means it accepts keystrokes but doesn't show it. There are two fixes:

Add these lines to your .bashrc file:

stty ixany
stty ixoff -ixon

Now, when you press CTRL+S you'll see this:

sjoerd@sles:~>
(i-search)`':

What this does however… I have no clue. I like it more to just add this line:

stty ixany

Now, when you hit CTRL+S the XOFF gets initiated but any character will start XON.