Rebolting - drilling angled bolt holes

FYI, ran into a scenario the other day while re-bolting: pulling original mixed metal 3/8 bolt and up-sizing to SS 1/2. The original hole on a few placements was drilled at a more than slight angle (unsure if this was intentional or not as the rock face was nearly planar at most placements). When up-sizing with 1/2 inch bit (newer sharp bit, modern rotary hammer drill with plenty of power) I tired to make the new hole perpendicular to the rock face. This resulted in two non-overlapping holes at the back of the new 1/2 hole effectively increasing the hole diameter. See diagram. 

As this was reducing the rock in contact with the new sleeve I ended up patching and drilling a new hole nearby to maintain full strength. 

This is more of an FYI and heads up to other re-bolters. Have others encountered this situation? Thoughts? 

Another reason to rebolt with glue-ins! 

If it's a slight angle you can typically get the entire original hole within the new hole. You do have to keep an eye out for more steeply angled holes, which is what you're describing. Occasionally you just need to patch and drill a new hole if the old hole can't be incorporated into the new hole. Unless (like Shawn said) you use glue-ins.

It's pretty common to see angled-in-to-the-left 1/4" holes drilled from stance, since the climber was gripped and they weren't paying attention to left-to-right angling (and most climbers are right handed). Luckily since 1/4" holes tend to be quite short, it's usually pretty easy to get the old hole within the new 3/8" hole (or super easy with a 1/2" hole). Longer bolts of course make it tougher to incorporate a slanted hole within a new hole.

+1 on the glue in