I'm using Vim 7.4.1825, huge version without GUI on OSX.
When I start Vim with Vim -u NONE, navigating to mark a - set with 'a - is not working. Every other mark (b-z) is working well, only jump to mark a is not possible. I can see with :marks that a bar foo is set, but I cannot jump to it. Is there a explanation for this behavior?
This is possibly an issue with dead keys, a way of using a "regular" keyboard to provide a way to generate diacrtics such as: á, í, or ó.
One way to test this would be to enter 'a in your terminal proper (rather than vim within your terminal) and see what character is produced.
If it does produce a diacritic, that means the terminal may well be getting to the keyboard stream before vim does, and all vim will see is the diacritic rather than the expected key sequence.
If that is the case, you could opt to switch to another terminal which better allows you to control this, or there is at least one solution here which sneakily map the diacritic back into the correct key sequence that vim will process.
'a? You can check with:nmap.:normal! 'awork?:normal 'a? If not, then maybe a mapping is interfering as @statox said (look at:verb nmap 'a). Otherwise, maybe something is intercepting the keystrokes'abefore Vim (your terminal emulator, your window manager, ...). See in their configuration if'ais not already bound to some function.