Hi,
In the product edit form, the first button in the HTML is the button for the quantity.
That button does not validate the form, it just call a javascript function set the product quantity to unlimited.
By pressing the "enter" key, your browser look for the first available button in the form and submit it.
For most cases, it submit the entire form but in that specific case, it just activate the product quantity unlimited button.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.