If you go over the $5k, then yes you'll have to pay taxes on the excess, but there is no penalty. For example, if you end the year with $8000 in contributions and you file claims to get reimbursed for dependent care expenses on that entire $8000, then you will simply have to pay taxes on the excess $3k when you file. Interestingly, you will not have to pay FICA on this money, only income tax (unless they fixed that loophole).
The only scenario which should really matter to you financially is if your husband's old employer will not reimburse your dependent care expenses from the FSA funds that they're still putting in there. They will still report their contributions to the IRS, and the IRS will treat these excess contributions as income, so you'll have to pay income tax on it, but if you never have access to the money, then they're just generating taxes for no reason, which is a big 'ol pile of bullshit.
Since you don't seem worried about that $400 from the old job, I assume that you can still use those funds. I'd either confirm that these continued contributions beyond that $400 are indeed free money that you really can use to get reimbursed for dependent care (wouldn't that be nice?), or make sure that they cease those contributions and remove any excess contributions from the account. I'd make these requests in writing (e-mail works) and in person so that I can keep a copy of the requests in case of an IRS audit.
I've worked in HR before and it's likely that someone just forgot to check a check box or send an e-mail and it slipped through the cracks. If your husband's old employer isn't a bunch of assholes then you shouldn't have a problem getting them to sort this out for you.
Good luck!