The phrase "occasional accident" means that something bad or unplanned happens every now and then, but it doesn't happen very often.
Sentences with «occasional accident»
Some trainers are meant for children just learning to use the potty, while others are made for children who are using the potty with occasional accidents. (the-cloth-diaper-connection.com)
I read that parents should expect occasional accidents even up to the age of 7 or so. (babycenter.com)
She will signal by the door when she must go out, but she does have occasional accidents inside. (anewstartonlife.com)