Under 2.7 (on pasta displayed remotely) rapidly clicking mouse buttons (1,2 and 3) on day, month, week and year view buttons, in dtcm with LANG=he_IL, gets the following XError:

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X Error of failed request: BadDrawable (invalid Pixmap or Window parameter)
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Due to long redraw (backing store delays) I guess it would be reasonable to assume that the latency in the Xm/Xt/X &/or complex text layout layers might be the cause.