Web Development

Hey guys! Sorry if this isn't the best place for this but I couldn't really find anywhere else.

I've been working for 6/7 years as a web developer full-time now, and I'm still plagued by one mega frustrating habit. When I'm working on something complex on one page, and it gets completed, I'll fairly often get notified either by the client or my boss a day or two later while they're testing the whole site that there's something broken on another page.

Almost always, it'll be down to the fix I've recently made.

Is there a way to avoid this kind of tunnel vision? I try to keep my code localised as much as possible, avoiding interacting with global scope and, if it's really for one specific thing, tying it down to that page in particular, but short of testing the entire site every time I make any change... is there anything else I can do?

Thanks!