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Before I go into this bug report, I want to make it clear that this issue is not the same as getting stuck in Anivia's Crystallize (although it might stem from the same problem).
Occasionally when Anivia casts crystallize, heroes can pass right through it (no they were not using ghost).
How to re-create the bug:
A friend and I did a few test runs and this is what happened. We went into a practice game where I picked Anivia and he picked Veigar. We were both on the same team against no one so it was just 2v0. The only items we had were regrowth pendants. Around 5 minutes into the game we both went top lane to try to recreate said bug.
My friend (Phreak) started right next to the tower at the top lane as Veigar. I, as Anivia started some distance away directly south of him. (Roughly 1.5 the tower's range) He then proceeded to right click our base (blue base) only once so that the path was set as a straight line to the fountain. As he was walking, when he reached about half way in between where he started and where I was, I cast a crystallize. I was careful not to cast it on top of him but rather in front of him. About 20% of the time, Veigar was able to nudge his way through the wall and walk straight through it. Other times he simply stopped walking even though there was a clear path to the fountain. The other times he behaved normally and walked around the wall back to the base. Note that Phreak only right clicked once to set the path and did not right click around the wall after it was cast.
Hopefully this bug gets looked at as Crystallize is a very unique type of skill only available to Anivia which separates her from other disabling heroes.