sounds like you havent closed a } bracket somewhere and the next line is an event catch.(definition)
if the } isn't there then the line is being included in the last bit of code, meaning mirc is trying to use ON x as an alias.
bad-
alias stuff {
echo -s this is inside the alias
echo -s the next line is where the curly bracket should go but it's commented out
;}
on 1:input:*:{ stuff }