Mum's hilarious Halloween fail makes her feel like the 'worst parent in the world'.

British mum Ashlee Myers and her son had an eventful Halloween. But uh, there was less trick or treating and more… head scrubbing.

You see, Ashlee made a wee mistake when putting together her son’s Halloween costume and took to Facebook to explain.

“Feel like worst parent in world,” she wrote. “Got hair spray from pound shop right with all Halloween stuff … Spray it on thought yeah looks ace that.”

"Finished up thought smells a bit strong... Looked at label ITS F-----G SPRAY PAINT I've fully dyed his f-----g head and chances are I need to shave his head coz after 30 min scrubbing I CAN’T GET IT OUT!!!!!" the post continued.

Oh... No.

The photo of the spray paint can said it was "highly visible" and "ideal for hazards and obstacles", with a "tough formulation" and "extra bright matt finish"... So, not exactly what you'd like to put on your child's head.

Because it's hilarious, her post went viral and had more than 40,000 likes within hours and also made its way onto Twitter.

Thankfully, her son was not permanently left with bright red hair.

Ashlee later shared an update to say she and her husband managed to get the spray paint off with a lot of elbow grease, experimentation and dishwashing liquid.

"Between me and my husband we managed to fix it ... washing up liquid is amazing stuff," she wrote in a comment on the post.

It all ended well - but we bet no one in Ashlee's family will be allowed to dye their hair anytime soon.