Sometimes development is so rote that you have to push yourself to be disciplined about testing properly. With the advent of browser-based development tools, and in particular the ability to test a site responsively, it is becoming easier to iterate and develop.

Unfortunately, the probability of something going awry is high enough that it always pays to double check how a mobile site looks on an actual mobile device.

Recently, I experience an issue where a couple of photos were not displaying properly, but only on an actual mobile device. Had we not tested properly, the issue may very well have eluded us until some time in the future. This would've led to an unprofessional looking site. Fortunately, someone on the team caught the issue and we were able to quickly find a resolution.

It's time-consuming and easy to get lulled into a sense of safety, but it's important to test often, test on various browers, and just as importantly, on various devices. Or if you have the means, utilizing a site such as Browserstack.