Watercannon as a plural: "The police used watercannon to dispel the rioters." Why not 'a watercannon' or 'watercannons'. They'd sound much better.
Carnival, when referring to the Notting Hill Carnival: "Over 200,000 people attended carnival today." Why not 'the Notting Hill Carnival' or 'the carnival'. That'd sound much better.
There's probably good english language reasons behind them but I don't care.
Any more that get YOUR goat?