Android Debug Bridge is often used by advanced users to flash files directly to their phones, such as system updates, but an ADB glitch that popped up recently seems to be causing the tool to crash when trying to flash large files, which is keeping users with qualifying Nexus and Pixel devices that have yet to get the Android 7.1.2 update over the air from flashing it manually. The bug has a thread in Google's bug tracking system, and seems to have first surfaced in the rev.25.0.4 update, which came out in March. Nexus and Pixel users are not the only ones affected by the bug; just about any device that can have a system image flashed to it via ADB can't do it with this new version, according to users in the Google Code forum. Even sideloading system backups made in TWRP is a no go.
The bug, as mentioned above, only seems to affect large files, so small updates like security patches should be okay. While Google has yet to issue a fix in ADB, users who are having the issue need only uninstall ADB and roll back to the December 2016 version, rev.25.0.3. While this version may not be as feature-rich and optimized as the latest for smaller transfers and day-to-day usage, such as for app development testing, those who find the large file bug to be a deal breaker have another option, at least. Those who want the latest ADB but also want to flash large files can always install the older ADB in a sandbox, virtual machine, or dual-booting OS on their PC; many users out there, especially developers, already dual-boot Windows and some variant of Linux on their PC, so using the older version on one OS and the newer on the other would be the easiest way for this crowd to get around the bug.
Another alternative is to use your particular manufacturer's flashing tool, such as Samsung's ODIN or LG's LG Flash Tool. Some manufacturers do not have such a tool, however; in this case, yet another possible alternative would be using Chainfire's FlashFire app on your device, provided that the device in question is rooted and there aren't any compatibility issues. As with any other instance of messing around with system files and risking bricking your device, proper research should be done prior to taking any action.