Mind spike, a 2nd-level spell included in Xanathar's Guide to Everything (p. 162), seems to use different damage dice for the base damage and for additional damage when cast at higher levels.
I have the First Printing of XGtE, which has the following text (bold for emphasis mine):
2nd-level divination
[…]
You reach into the mind of one creature you can see within range. The target must make a Wisdom saving throw, taking 3d8 psychic damage on a failed save, or half as much damage on a successful one.
[…]
At Higher Levels. When you cast this spell using a spell slot of 3rd level or higher, the damage increases by 1d6 for each slot level above 2nd.
I have not been able to find any other spells that use different damage dice sizes (d4, d6, d8, etc.) for the damage at the base level and the "At Higher Levels". (The only quasi-exception are spells like ice storm that that use multiple different damage dice at the base level for different damage types, but in this case the damage at higher levels uses the same die size as the matching type of damage at the base level.) So my suspicion is that this should do base damage of 3d6 or additional damage of 1d8 for each slot level above 2nd.
Has the mind spike spell's damage been errata-ed, corrected in later printings, or is there any other evidence that this is a misprint?
Best Answer
It was a typo
Prior to this Crawford confirmed it to be a typo and the spell entry on D&D Beyond showed 1d8 added per level (as one would expect). With the release of the 2020 Errata to XGtE mind spike was corrected:1
You may also be interested in this effort on the Enworld forums to document the typos and errors in Xanathar's Guide to Everything.
1: I'm not entirely clear on which printings have this correction. The errata document says fourth printings and out have corrections, however it does not appear to be fixed in the 5th printing, which is the latest information available to me.