'Drinks on me!' (First of several errors) (4-3)

'errors' is the definition.
(mess-up is a kind of error)

'drinks on me first of several' is the wordplay.
'drinks' becomes 'sups' (I've seen this before**).
'on' means one lot of letters go next to another.
'first of' says to take the initial letters.
The initial letter of 'several' is 's'.
'me'+'s'='mes'
'sups' put after 'mes' is 'MESS-UPS'.