I'm having a frequent issue (like, several times per minute) where I'll type a key and see no feedback from the OS (whether in the browser, terminal or otherwise). The key does seem to have been registered though, because the very next key pressed appears to then deliver both missing key and the one just pressed. There doesn't seem to be any consistency to which key causes a problem; I've noticed on this letters, enter, space bar, etc.
I'm running Ubuntu 15.04 via VirtualBox 5.0.14r105127 on Windows 10. Keyboard is the standard English (UK) and is the only keyboard registered. VM has 6GB RAM, 16GB HDD (of which about 8GB is free) I'm not running anything intensive- just the browser, terminal and a couple of small utilities.
Disclaimer: I'm an Ubuntu/Linux noob.