There are some CSS shorthand declarations that require the use of
/ which LESS may interpret as the math operator for a division (depending on the values in your declaration). Examples for this are:
font: font-style font-variant font-weight font-size/line-height font-family;
background: background-color background-image background-position/background-size background-repeat background-attachment;
I ran into the latter use case while defining multiple backgrounds in shorthand. You can escape the slash in LESS using
background: url("../images/bg.png") center 300px e('/') 70% no-repeat;
Alternatively, there is the option to enable "strict math" in LESS; when strict math is enabled, all math operations must be wrapped in parenthesis (10px / 5px) which causes overhead there, but solves the slash ambiguity for shorthand.
Choose your poison!
PS: Obviously you can also avoid this by not using shorthand for cases like font and background.